Thanks for your answer.
I understand your point about the solver, and your proposal for fixing the problem using imposed displacements is quite ingenious. However, how do I know the importance of the displacements and where to put them? I fear the results I would get wouldn't be rigurous enough.
isn't there another way of modelling the pontoons bridge behaviour that wouldn't require that? I just need to have central pressure on the vertical axis (passage of vehicles) and hydrostatic pressure counteracting, while the perimeter is not fixed (because in reality it's the case).