If the runway and taxi ways were marked with floating cone's or buoy's. To ensure you stayed on the narrow path. And
If you built a small jetty or similar. And
If you applied the same texture, specular and bump mapping to runway and taxi way. And
If you had a ground plane with the above maps etc, scaled of course. And.
Well that's it really, you should have a fully working water landing strip. Maybe a few tweaks. And a big AND.
If someone developed an aircraft with floats. As 3rd party aircraft development is nil as far as I can see, I wouldn't hold your breath, Scuba gear needed for that.
PS, you could even add little lights on your buoys or cones for night time landings
Seaport of Vancouver Intl.
Not exactly what you described, but what can be realised at the moment
As jetpack wrote some while ago:
The physics of AFS2 currently did not implement swimming objects.