This is the next step after the sea plane. I found a thread with some one asking for a car that can drive under water, well here ya go. the only thing is for the version 1.0 you have to press the stunt button in order to get out of the vehicle.
Update 2.0- More cars can drive under water now. Still have to use the stunt button to exit the vehicles. As far as I've tested none of the motorcycles work. If you find a car that doesn't work or have any ideas on improvements to this mod let me know here http://www.justcause2mods.com/index.php/forum?func=view&id=6530&catid=16#6530
BTW the file is too big to upload here so the download button will provide a link to the file.