have you thought about using resin?
instead of an actual sheet, you get two part resin (I do not recommend something like woodland scenics for large areas, it's better for bases and small uses
imo), mix it, add your tint (washes work, like
GW, Liquitex tints, inks etc) and pour onto your surface.
Be very very sure to cover places your working on, as well as make a waterproof framework to keep the resin in, so it doesn't leak out. Some leaks seem inevitable
tbh, but overall you don't want all of it to leak out or you end up starting over! I ended up using hotglue and some boards, when I did my
swamp board, a LOT of hotglue.... it's good because it comes off easily and applies easily. As well as being able to contain leaks well. I threw a cheap plastic table cloth cover down for the inevitable leaks/miss and it worked very well.
Alternatively, you could get something like those clear rug mats maybe, or even glass, paint the board like water, put the glass/plastic cover on top.
You might even just print a picture, or get a print of water, and do similiar.
Anyway, just some thoughts! Good luck!