It has been suggested before, but a challenge or campaign level set on open water would be interesting for a Marine park.
The concept is a bit extreme, but I think worth exploring on paper at least.
I'm thinking square pieces of boardwalk would function as ground and path, with the boundaries behaving like lagoon sections, having gallery attachment points, attaching to each other and merging where appropriate.
The entire map could be a grid for Boardwalk sections, so it'd be possible to place anywhere, even unconnected and connect up again.
For ease of use, a few different sizes and shapes of boardwalk would be good, but would be best to have them be multiple pieces conjoined to make deleting simpler.
Net fencing attached to buoys for the animals probably makes the most sense, but Concrete or Rock could probably work too. Being able to place fence under the boardwalk might also be interesting.
You could have new feeder variants as their own floating buoys, or maybe just attach them to the boardwalk or fence.
Preferrably Aviaries and land exhibits would be possible on the pier sections in sandbox.
Different boardwalk types would be good, but the classic wooden boardwalk is what I'd argue for, even being destructible by the larger animals might be interesting.