I've been dialing in swamp puddles with Vallejo water effect gel. Left is 4th attempt (wetblended brown gradient that didn't come out that great and 1 drop of green paint in the water), right is 5th (a really smooth dark blue to brown gradient completely hidden by skipping to 3 drops).
Current conundrum is the cracking you can see, which is probably just me pouring too thick a layer? 'Bout ready to do a larger piece.