Grounding a 2 gang metal box in a wet area
The fact that it's a wet location doesn't make a difference to how you make your wire connections (although it needs to be on a GFCI circuit because it's for a pool light). Connect a pig-tail ground wire the green screw on each of the switches and a 3rd one to the green screw on the box.  Wire nut all three of them together with the ground coming into the box.