You have couple of options to account added water mass.
Simpler one is to use MFLUID region as shown in this video.
More resource consuming method is to use specific elements that represent water.
In both cases check Nastran documentation file adv_dyn.pdf located \Femap_Directory\nastranhelp\pdf\. Look at chapter "Dynamic Analysis with Incompressible Fluids".
If you need some test case to check if you have correctly applied virtual fluid mass than I recommend to look at this paper. it has results simple cantilever plate partially submerged in water. This experiment can easily recreated in Femap.
I didnt have problems with plate elements, but i want to know if i can use solid with fluid regions. I only get troubles, maybe only works with plate elements.
Made two simple test models, lets say cantilever plate, one meshed using plates and another using solids. Run them without fluid region and with fluid regions with same setting (height, density). Compare results and you will see if both meshes produce close frequencies both in "dry" and "wet" configurations.
