Alright, here's my story. I have an autistic little brother who is desperately in love with building things in creative mode. He built this ridiculously large cruise ship. He told our parents that all he wants for Christmas is for his ship to move. Our parents, being not so tech smart, called me up and said "Hey, you're into computers right? Can you get this to work for your little brother for Christmas?" - I'm in college for mathematics and some C++ programming on the side... I played Minecraft like five years ago... I'm far from modding anything.
So! What I'm looking for is for someone to point me in the right direction of an already existing mod, or set of mods, that I could use to get a pre-existing set of blocks to float on water. I'm pretty out of date when it comes to Minecraft, but 'back in my day' this was pretty much entirely impossible. If this is stupidly easy, and something I can do on my own, I'd love it if someone would tell me how to stop being oblivious to the obvious solution. Alternatively, if this is actually something ridiculously difficult to accomplish, I'd go so far as to hire someone(s) to write up a mod for this specifically (is that allowed on here? If not, nevermind, lol). If I'm in the wrong area entirely, then someone point me in the right direction! I need this damn boat floating by Christmas.
Having an autistic younger sibling myself I can relate to your situation and am happy to help. Lucky for you there is a pre existing mod that should allow his ship to not only float but even fly if that's his wish. The mod you'll need is Archimedes Ships. http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/1289952-archimedes-ships-v1-7-banking-ships By placing a "Ships Helm" block on the ship you can turn any structure into an entity capible of movement. You can add "Floater" blocks to increase the ships buoyancy, "Steam Engines" to increase top speed, "Passenger Seats" to allow other players to ride along, and finally "Air Balloons" which will allow for flight. You may need to adjust the config file to allow for larger ships or to increase the effectiveness of the "Floaters" and "Air Balloons", so as not to ruin his design with too many new blocks. Before assembling the ship into an entity be sure that it is not connected to anything such as the ground or a dock.