That's what I was going to say - if I hadn't already - You only get ONE shot at this when you use this stuff, so maybe make a small, scaled-down version of what you're doing, paint it the way you intend, do the beach, etc., fill with Woodland Scenics water, let dry, etc., then see if the acrylic gel medium on top of that will work for waves.
Because if, for some reason, it doesn't, you are well and truly screwed if you've done that on what's shaping up to be a magnificent diorama display.