have you considered making the cake as the water part and then making the boat out of rice krispy treats?
Honestly, I would just make a bunch of sheet cakes and stack them together and carve the boat out. I don't think that the boat would be that hard to carve.
I have no idea how many servings it would be. What I would do to figure it out, is just add up how many sheet cakes that you use and multiply that by how many servings the one sheet cake is supposed to give you. Then you will have a round about answer. Don't forget that you're going to lose some in the carving process. Use that to make sure that you will have enough cake.
I do know that you are supposed to charge for ALL the servings, even though you will be throwing away a lot of the cake during the carving process.
I hope this helps!
Yo can see the Pirate Ship on my galery I think you can carve it like mine on the front and just change the back, if you want I can help you how to make the water and the carving, I will love to help you, I hope it is not too late