The problem with the aiwha is since it is an animal, it would probably require new animations for the wing movement, most vehicles on the other hand are inanimate requiring little animation. But yes, it would be great to ride around on one.
I think an underwater vessel on land should catch fire like a swoop after a time or generally not move, but instead be only usable underwater. Maybe even the ability to push the vehicle into water terrain would be fun.
There is a more advanced path of going into coding, but at the moment I think even a ship that looks like an submersible vessel would go down great, even if it could fly through the air too.
It is a shame all nautical aspects of the game have not been improved; animations, vehicles, abilities.