There are three distinct areas within the general outline of Nedonna Marsh and Woods.  The smallest segment is a continuation of the sand beach and dunes that fall under state jurisdiction in accord with Oregon's beach law.  The Tillamook County Pioneer Museum acquired the most critical ecological area that includes the salt water marsh and forest to the east as a result of a donation by Mr. Loren Parks.  The Museum is committed to preserving the marsh and woods  as a natural area. The remaining section is owned by a development company.
Neither the small segment controlled by the state nor the critical area controlled by the Tillamook County Pioneer Museum can be developed. 
What development is possible in the remaining section? That is a simple question without a simple answer. What development is allowed on a parcel of land is governed by its zoning. All private land in Oregon is zoned for particular uses, which allows some kinds of development and prohibits others. The zoning in this case is done by Tillamook County, since all of Nedonna Marsh lies outside the City of Rockaway Beach's city limits and also outside its Urban Growth Boundary, which is the land that Rockaway will be allowed to urbanize in the future.

Nedonna Marsh is restrictively zoned, as is most coastal shoreland property, due to its flood hazard potential and the high likelihood of tidal erosion. However, even though housing development is therefore unlikely, development threats remain. A developer could propose a campground, trails and kiosks or some kind of small scale "eco-resort" that would ruin the Marsh's ecological integrity.
Nedonna Marsh's zoning is especially complex. Like all property, it has a base zone, which tells you what development is allowed. But it is also subject to various overlay zones, which restrict activities that would otherwise be permissible in the base zone. This is common for sensitive lands like coastal shorelands, wetlands, riparian zones, and estuary edges. As Tillamook County explains in the zoning code, "An Overlay Zone is a supplementary zoning designation placing special restrictions or allowing special uses of land beyond those required or allowed in the Base Zone." 
The base zoning on most of Nedonna Marsh, both the SeaRiver and Loren Parks ownerships, is Recreation Management. Part of the northern end of the Parks property is zoned Estuary Development. The base zone tells you what developments are allowed, either outright or with a conditional use permit. RM allows, among other things: recreational campgrounds and group lodging facilities like dorms, restaurants, golf courses, and single family residences (on a legally-created parcel at least five acres in size).
But that is only the beginning. The minimum lot size in the Recreation Management zone for land divisions and development is 40 acres. A single family residence built in that zone must be on a legally created parcel that is at least five acres in size. The Director may approve a smaller lot size under certain conditions. Other kinds of development in RM like golf courses, restaurants, campgrounds etc., probably have a 40 acre minimum lot size.
Thus for example, if SeaRiver had a legally created five-acre parcel in their 22-acre ownership of Nedonna Marsh, they still would not be able to build a house, because they don't own 40 acres, and 40 acres is the minimum lot size in the Recreation Management zone.
However, that is not the end of the story, because there are several overlay zones that also apply to Nedonna Marsh. Overlay zones affecting Tax Lot 109 (SeaRiver) and Tax Lot 100 (Parks):
1. Freshwater Wetlands
2. Flood Hazards
3. Shorelands
4. Beaches and Dunes
Here are some notes on each of these zone overlays to help navigate their details:
1. Freshwater Wetland Overlay: for any uses that require a permit in the "notification wetlands" (probably including the wetlands of Nedonna Marsh, if shown on the Statewide Wetlands Inventory maps), the Dept. of State Lands must approve the proposal as well as Tillamook County. Significant Goal 5 wetlands are mapped by Tillamook County; notification wetlands are mapped by DSL. Generally, uses are not allowed which "would destroy or reduce the biological value of the wetland."
2. Flood Hazards Overlay: Tillamook County adopted the FEMA (Federal Emergency Management Agency) definitions and basic map delineations for flood zones. The accurate map is on the FEMA website; Tillamook County maps are for reference. The definitions for AE, A0 and VE flood zones are in the Flood Hazard Overlay Zone definitions (on p. 5).
The purpose of the Flood Hazard Overlay is, among other things, to maintain existing floodplains; controlling/minimizing development which would increase flood damage or hazard; encouraging mitigation and restoration; prohibiting uses dangerous to health, safety and property due to water or erosion; or that lead to increase in flood heights or velocities.
3. Shorelands Overlay: Which properties are covered by the Shorelands Overlay depends not on any maps, but whether the definitions fit the property. Parks' property certainly meets the definition in the Shorelands Overlay, 2 (a)(1), (3) and (4), which are lands subject to ocean flooding, within 100 ft. of ocean shore or 50 ft. of an estuary; riparian vegetation necessary for shoreline stabilization; and "significant shoreland and wetland biological habitats." The Parks property also meets the dune definitions in 3(b)(1), as significant shoreland and wetland biological habitat.
The SeaRiver property also meets the Shorelands definitions in 2(a)(1) and (4); also probably 3(b)(4), which includes conditionally stable foredunes and interdune areas subject to ocean flooding. SeaRiver's ownership may also fit the definition in 3(b)(1), significant shoreland and wetland biological habitat.
Generally, only low intensity uses and developments are allowed in Significant Shorelands -- if the base zone allows any development, no major impact to any significant wetlands is allowed.
4. Beaches and Dunes Overlay: The Parks property is likely Category 3 (Beaches, active foredunes, conditionally stable foredunes, interdune areas). The SeaRiver property is probably Category 3 also (Conditionally stable foredunes subject to ocean undercutting or wave overtopping), maybe partly Category 4 (younger or older stabilized dunes).
Generally, development is permitted on stable foredunes if not subject to ocean undercutting or wave overtopping. No filling/draining of significant deflation plain wetlands is allowed. A Dune Hazard Report to Tillamook County is required, in general, before any subdivision or partition.
Development Options: Section Line Road is the boundary of the Rockaway UGB in this area. Thus, all of Nedonna Marsh is outside it. Development could only be possible in Nedonna Marsh either by (a) having the owners petition Rockaway to be included in the UGB so they could get sewer and water; or (b) applying for a development through Tillamook County Community Development Dept.
Since the Marsh is currently in Tillamook County jurisdiction, developers would probably find it easier to apply to the County for the necessary permits. However, as there is no sewer in the Nedonna Marsh area, any development would be on septic systems. Water would have to come from groundwater wells. It would be very tough to meet all the restrictions found in the Tillamook County overlay zones for housing or commercial development on the SeaRiver property, given the clear zoning effort to restrict development in these locations for public health and safety and environmental reasons.
 Becoming part of the UGB would be impossible. UGB expansions are based on need, and must be justified by calculations showing housing trends, population growth, household size, commercial and industrial land inventories, alternative lands that would meet the expansion needs, and other factors. The SeaRiver property would have no grounds to be included in the UGB, as Rockaway Beach does not need additional housing and could not justify a UGB expansion in this area. Even if the Marsh were brought into the UGB, similar health and safety restrictions limiting development would apply as currently do under County zoning.

How can Nedonna Marsh be fully protected? The best solution would be public purchase or donation of both the SeaRiver and Loren Parks parcels. Owned by the State of Oregon, Tillamook County or another public entity, the Marsh would be overseen by the public. If any restoration or enhancement project were indicated, such as culvert replacement for salmon habitat or riparian vegetation plantings, public monies could be easily applied for. Friends of Nedonna Marsh supports full public acquisition of Nedonna Marsh. One of the owners, Loren Parks, is on record as desiring to give his share of the Marsh to the public for permanent conservation, as stated in a 2007 Willamette Week newspaper article: "Parks said he bought the land to give it to the state…'There's a lot of people who go down there,' Parks says. 'They watch the boats, they watch the surf, they watch the sea animals, and all of that.'"
Appendices:  Tillamook County Zoning Documents
Recreation Management Zone Freshwater Wetlands Overlay Estuary Development Zone Shorelands Overlay Zone
Flood Hazard Overlay Zone Beaches & Dunes Overlay Zone Marsh Zone Maps