If it fits in a washing-up bowl, put enough water in the bowl to float it, put hull in water, push down to waterline, mark the water level on the bowl side, take boat out, then get scientific. Very carefully, pour measured amounts of water into the bowl until the mark is reached.

The poly under the hull will raise the centre of gravity. Raise it enough and it will fall over. (Watched some Round Tablers rehearsing for a raft race with a bath modified like that. NPML

)

For a rectangular boat, its just the floor area times the depth to get the volume. The bit at the front has a triangular cross section, so the volume of that is easy enough.Its just a question of sticking formulas for each chunk into a spreadsheet and adding the answers together.