If you have an aquatic army, basing them on earth and green looks ... peculiar.  Incorrect.  But how do you make up nice aquatic bases?

It's easy; here's how.

The hard part is to get clear acrylic bases, 3mm thick (1/8"), cut the right size.  I've got a plastic cutting blade on my tablesaw, which works, but is still scary.  Laser cutting is better, if you can manage it.

Once you've got the acrylic bases you need, buy some sheets of Plastruct "Stormy Water".  And get some E6000 glue.  That's a brand name, not sure what it actually is, but you need it -- it is transparent and it will glue the Plastruct to the acrylic permanently.  Crazy glue (cyanoacrylate) doesn't work, it frosts the Plastruct.  Contact cement isn't clear.  Most other glues won't glue acrylic.

So take your bases, smoodge them with E6000.  You don't need to completely cover the base, but mostly.  Air bubbles look fine under the Plastruct, so don't fuss them too much.  Press the bases down onto the Plastruct.  Let dry.  This is hard -- it takes many days (at least three) because there is no air leakage.

Then trim the edges of the plastruct to the edge of the base.  I use sharp scissors to get fairly close, and then an exacto knife for the final trim.

Figures are just glued to the surface using the same E6000 glue used to affix the plastruct to the acrylic; it dries clear.