Well the rules come from Archimed, with the formula p = P*V*G (buoyancy)
were G is the gravitational constant (about 9.8)
V is the volume of water displaced (the volume of the part of the ship in the water, it is in meters cube)
P is the density of the fluid (of the salt water in this case)
Concrete boats have been made, check my source for some examples.
If you want to make your own (big or tiny), here are some tips:
The thinner the concrete, the better it is.
Add weight slowly, too see if it will hold or not

Cheers!
Source(s):
http://en.wikipedia.org/wiki/Concrete_ship

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