At least some of these fish see through regular invis.
I was running (levitated) over the water with a fresh invis, when one of these darling little fish starts hamstringing me for 93 points! That was one of the rare times I needed help on a corpse run. It was behind a rock far from shore, with these fish swimming around it. Cammo type spells DO work, thankfully.
Just something to keep in mind when you want to take a swim through the tunnel to the hidden area.