Sorry, forgot to add that the additional load on the top of the deck includes a rain load.
I didn't look at the model but symmetry would take out the in-plane motions. As far as the vertical directions go - you could juts add a vertical restraint to the surfaces that contact the fluid as a first approximation to simulate the resisting pressure. Probably close enough. Otherwise you could compute the pressure distributions on both side and ensure that they are balanced and then use inertial relief.