Konstantine Kourbatski
Ansys Employee

The full electric displacement in Maxwell's equations is:

D = eps * E + P

where P is the polarization current density which comes from the change in polarization of the individual molecules of the dielectric material. Fluent's MHD drops the P term and defines D as:

D = eps * E

So the question is if seawater is a conductor enough at a given salinity that the polarization current density can be neglected. Depending on your needs and fidelity of results sought, P in sea water may or may not be a factor. 

This is a CFD forum, and my knowledge of Electromagnetics is somewhat rusty. I suggest you post on the Emag forum, I recall people working with our HFSS have looked the effects of salinity on sea water polarization, and they may give a better guidance.