Hmmmm.... what are you doing on the cake? Brown sugar can always be used as "sand".... you could use that for the cake base board (maybe make it a little bit larger than you normally would), drape blue satin over a regular cake stand to replicate water, and have seashells placed on the "sand", "water", and table with some pretty votives and tropical flowers.
For my sis-in-law's beach-themed wedding cake, I bought a few pieces of tulle in different blue shades. I draped and fluffed them to look like waves, and put the cake on that. Surrounded it with some sugar "sand", and placed a few handmade chocolate shells and the beachy-looking glass beads you can buy at the dollar store.
I prefer crushed vanilla wafers as well for sand. There are a few options for the "stand" as well if you don't want to use a standard one. You can place a cake on large mirror or glass top to have some reflection properties, use glass blocks or heavy candle holders for risers. You can also use a crate box for a stand or a nice rustic board on risers, but that may take some doing or searching out.
That's true, or you can actually use those square clear glass vases for risers too and fill them up with flowers in water or colored water or even put a gold fish in them! That would be cool if the cake is not too big or heavy--cannot imagine the mess if those things didn't hold