Another element with a full mass matrix capability is MATRIX 27, In the help looks for: help/ans_elem/Hlp_E_MATRIX27.html
You can always add an APDL script in Mechanical as well. Check out this video:
I am not sure of the scenario you are dealing with but if you are looking to work with Ocean Loading from a Hydrodynamic Analysis, check out Chapter 13 in the Mechanical APDL>Advanced Analysis Guide.
Lastly, If you just want to apply fluid pressure in Mechanical, Follow these steps:
- On the Environment context toolbar: click Loads>Hydrostatic Pressure. Or, right-click the Environment tree object or the Geometry window and select Insert>Hydrostatic Pressure.
- Define the Scoping Method. Hydrostatic Pressure can only be scoped to faces.
- Select all of the faces that will potentially enclose the fluid.
If you are working with a surface body, specify the Shell Face, defined as the side of the shell (Top or Bottom) on which to apply the hydrostatic pressure load.
- Specify the magnitude and direction of the Hydrostatic Acceleration. This is typically the acceleration due to gravity, but can be other acceleration values depending on the modeling scenario. For example, if you were modeling rocket fuel in a rocket’s fuel tank, the fuel might be undergoing a combination of acceleration due to gravity and acceleration due to the rocket accelerating while flying.
- Enter the Fluid Density.
- Specify the Free Surface Location, defined as the location of the top of the fluid in the container. You can specify this location by using coordinate systems, by entering coordinate values, or by clicking a location on the model.
- Mesh the model, then highlight the Hydrostatic Pressure load object to display the pressure contours.
Hope this helps.