Just to reiterate what Tilt said... I see good suggestions for the role of float plane, but some of them require additions to the gaming concept, so I think we should keep that separate from the reasons why we should add these planes to AH.
My personal feeling is that having a float plane like the Emily or the Catalina as another goon or bomber/torpedo plane gives more strategic values to ports or vehicle hangers near the shore. This alone is a good enough reason for their addition I think.
Things like reconnaissance, pilot rescue, anti-submarine warfare, and the like can be added to the plans when HTC decides to add these features to the game. (I've seen good suggestions like for reconnaissance, wipe off all enemy facilities and airbases off the map and make them appear only after they have been photographed and the like...)
Even the addition of a float plane now, HTC needs to address certain issues. For example, why does the PT boat explode when hitting the shore at a very low speed? This would apply to the float plane I think. i would not want my plane exploding just because it bumped the dock. (while a CV steaming at full speed will drag your plane without killing you...)
HTC will also need to consider how we are going to turn these planes around after refueling. (well, this is not a hard task I guess, just like handling a GV...)
HTC will need to add boueys to mark the "runway" on the water and to identify "re-arm pads".
Of course, please consider making these "runways" unusable when there is a storm.

Ah what else... catapult launched seaplanes... addition of another type of task group without the CV will be interesting... perhaps more mobile than the current TGs...
Can we consider addition of submarines capable of launching planes?...

Just out of seer curiosity, kinda wanna see the I-400 and the Seiran modeled... hehehehe... Well, that is not the all planes that was launched from a sub... gotta do some research to confirm it though...
Ah well... just my ramblings and two cents on this matter...