concrete kayak/canoe

Gilad,

You will need to determine the thickness based on two critria.

Bending strength - bending moment considering in and out of water loads. Use the section modulus vs moments at various points along the structure vs the tensile and compressive strength. Keep in mind that kayaks are often carried from the ends and dont forget buckling under compression.

Hydrostatic pressure - with safety factors to take speed, and external loadings into account. Look in naval architecture books to determine what these should be for a small boat.
Alternatively back-engineer it from a typical kayak composite construction, which would be 6mm cedar core, with 200gsm Eglass cloth inner and outer faces.
By this I mean calculate the reinforced concrete thickness that will give you the same pressure bearing strength as the cedar/glass composite.

Dont forget the point loadings that come from standing on the floor as you get in. This will be much higher than the general hydrostaic load and may require some local reinforcement.

Good luck