Hello Tom (@hl68fx),
This should be an interesting simulation to carry out. But I’m skeptic about the displacement ranges we are talking about. Kindly have the post processing results in the project as @AnnaFless had pointed out.
6-DOF uses the mesh morphing technique, which is limited by the displacement of the body. Especially for your simulation the upthrust provided due to buoyancy might be very high to push the plate, resulting in a large displacement in a very short time span.
Ideally you can try to setup your model exactly like the Free Floating Boat in a Wave Tank simulation. Please start with a smaller immersion depth for the plate and make sure to have smaller time steps to get a stable solution. This can give you an idea about the displacements range in question. Then proceed with higher immersion depths, I’m not really sure about the stability at larger displacements. May be we can try to help you troubleshoot the problems for this model