OK - I pulled off the top of the deck lamp junction box and isolated
the wires going directly to the underwater bulb (recall that turning it
on trips the GFI, which tests to be working correctly.

Between the two leads to the bulb (green and black) there is
continuity. Between each of the individual leads and the pool ground
(electrical junction box) there is also continuity. I think this is
expected from the green (ground) lead --- but I'm guessing the fact
that the black lamp lead is also grounded is what's causing the GFI to
trip.

Is this right?

What's the next step in repair? Pulling the wire to the niche and
replacing --- or is pulling the lamp housing up the surface for
inspection next?

Thanks for ideas?