Try this, it's cheap and non permanent if you mess up.
Before you start, close your eyes and imagine what it would have been like to be there. What's wet.
Anyone fall, any equipment dropped in the water?
1- Figure out the position of the boat on your base and mark it out.
2- Higgins boats don't draw much water, but loaded they do settle down a bit. If you put this in an old picture frame you have a nice looking wooden base to contain all your work.
3-Build the base of you water area with simple styro-foam. You cut it close to the outline of your boat and snuggle it down on the base with some Elmers Glue.
4-When the foam is secure, make a batch of paper mache out of cellu-clay. Mix it all up and pack it around the boat, use a plastic knife to work it around the boat. You can make waves with your fingers if you want but a lot of landing were near or in a lagoon whenever possible to avoid that stuff.
If you think your sand will fit in now is the time to put it into the display. It will show in the shallows and won't as the water drops off.
5-Rough up the water around the edges of the boat and a good navy coxsin will keep the motor running so make sure the area around the tail of the boat is all roughed up.
When the paper mache is dry paint it any color or combination of colors you want. When you have that done get some DeCoupage Paste from a local craft store. Paint it over the water you painted.
When that's dry do some more painting and repeat the process again. If you do this two or three times you get a nice layered affect. When you are all done, get a tube of Window Caulk Sealer, White, tint it with a little water based color of your choice if you want and dry brush this along the edges of the boat, around the legs of your marines and everything else in the water.
Finally when you think this is all finished I have one more step for you to do Paint the Marines legs bodies and such and the interior of the LCVP and ramp with a couple coats of Future Floor Wax.Treat the wax like water. If your LCVP is tilted the water will pool to one side.
When you are done. Everything that needs to be wet will look wet. You won't be bogged down with Resin and when you want to clean the diorama, (Do a good job and put it in a case), wipe it down with a damp Kleenex.