Ed writes:

The picture I have
of these things is there is a red
brass conduit attached to the back side of the niche "can," and the
cord of the light assembly feeds through
it to the junction box. I've also read that the light assembly is
totally surrounded by water all the
time, and depends on that for cooling.

Yes, that's called a "wet niche" design. There are also "dry niche" types.

So, I have been wondering what
prevented pool water from
leaking like crazy through the conduit and into the junction box. But
perhaps that's where the polyurethane
sealant is used?

Right. No reason you can rewire it and recaulk it.