You can have a plane, AI or player, take off or land from any point on the planet, land or water. I cover that in detail in Chapter 15 of my Mission Building Handbook. The problem is that you may not get what you're looking for when using an AI plane.
AI behavior is inconsistent at best. AI Seaplanes should land "normally" on the water. The biggest problem you'll probably have is the plane overrunning the landing spot. In my experience that hasn't been too bad with seaplanes.
Trying to get an AI land-based plane to land (ditch) on water is trickier. Just setting up the aircraft to land on the water will result in the AI plane lowering its gear and landing on the surface of the water the same as if landing on solid ground. The AI plane won't sink into the sea, but will roll across the surface of the water just like it was on land. Replacing the contact points with those from a seaplane will probably result only in the AI plane starting in the air with its gear down, then landing on top of the water as I described. The pitch of the plane may vary, but it won't sink to the fuselage as you would want. At least it never has in any testing I've done so far. Also, the AI engines never shut off, even after the plane stops.
Another odd quirk is that the AI planes will taxi right through any infrastructure in the area. So if your AI plane "ditched" close to land, it would roll over the surface of the ocean and then roll right through any ships you had set up or right through a dock or wharf. Should it reach land, it will roll overland, up and down hills, through buildings, etc. until it decides to stop. I've even seen stock AI planes land on water, pitch over onto their nose, then run along the ground with their prop spinning into the ground for miles without stopping or showing any damage. As I said, AI behavior is inconsistent at best.
I like the idea of your mission. I've done something similar in a campaign but stretched it over two missions. In the first mission I set up a goal of ditching safely at the end of the mission. If the player accomplished that, he advanced to the next mission where he was now flying the PBY to rescue the fliers from the plane he ditched the mission prior. In that second mission I used the liferafts and signal flares, then required the player to land within a specific distance of the rafts, wait a prescribed amount of time, then takeoff and return to base.
If you or anyone else figures out a way to get AI planes to belly-land on the ocean, turn off their engines, and stop in a reasonable length I think we would all benefit.
Good luck and keep us posted.