Erik Kostson
Ansys Employee
See the keyoptions for mesh200 - 7 is for QUAD8 - there are other options for 3D, say 11 (HEX20). Never used it though so can not help with that - see emodif command how to change an element from one type say to another ( also this one will help:

Just to say that this analysis is wrong so there is not much point trying to do this, as this will take lots of time to set up and understand, and then it will not be correct (buoyancy and time evolution of a floating structure is not only a function of the force, but is also influenced to a very large degree by the hydrodynamic added mass and damping from the surrounding fluid ).

As I said if you want to do this , the only correct way is to use a hydro-panel method approach like we have implemented in our hydrodynamic ansys-aqwa software.

All the best