If you'd like to try it yourself, I used some Carbots timber varnish and simply added in a drop or two of my airbrush paints and gave it shake before dribbling it onto the board. Nothing to difficult about that - actually the hardest thing was waiting 12+ hours for it to dry.
These particular boards just need some more painting - maybe another hour's effort? - and they're done. But that water effect - it's pretty damn good for the price (effectively nothing this time around, as it was leftovers from some house maintenance-type stuff). If I was to try it again, I might attempt to paint some more depth into the pool first, thereby making it look even deeper... that could be a fun project to try one day, maybe even with some of the remaining boards. Watch this space.