Hi foggy, there are a few things I know concerning pool lights. On new installations, where there is a plastic niche, it is a requirement to have a #8 ground wire inside the conduit with the new pool light cord. If it is a metal niche with plastic conduit the same arrangement is required. If it is a metal niche with red brass conduit, this internal ground is not required since the conduit is an acceptable ground. It also needs to be sealed to the ground terminal with an acceptable potting material to preserve the connection in corrosive conditions.
The pool light cord should be pulled into the conduit leaving enough cord to be wrapped around the fixture that will allow removal of the fixture and lifting it out of the water for a bulb and gasket change. When is it pulled in leaving enough for the bulb change, coil the cord around the back of the fixture and place it in the niche catching the bottom of the fixtures tab into the slot of the niche, or plaster ring--then tilt it up to line up the screw with the top of the niche,or plaster ring.
Of course, the circuit should be protected with a gfci and incidentally, the pool or spa light circuits should have NO OTHER circuits in the conduit feeding the j-boxes--a common violation.
I have not seen a pool light that didn't have a minimum depth requirement of 18", but I dont know why 18" is the magic number. Also, they seem to not be designed for mounting on a floor, since they come with instructions stating "wall mounting only"