Nobody has tackled this yet, so here goes. Yes you have to break or sawcut or tunnel under the deck to get to the niche on the backside.
Where is it leaking? Around the faceplate? That's the part with the screws on it, on the inside of the pool. Or, is it leaking where the conduit[the part the wire goes through] is screwed into the back of the niche?
Surely they caulked both places. And it still leaks?
The colorlogic is a sealed light fixture. The leak in the niche would not affect the light it's self. There are replacable parts inside, or if it's in warranty you might get Hayward to replace it. Worth a try.
Here's the bad news. [there's bad news?]Yes there is bad news. Do these guys work on fiberglass pools alot? These pools are very tricky to work on. You don't just drain the pool. If the ground water is high, the pool could float. Yes float, as in out of the ground. I wouldn't drain it all the way. Just below the light niche where the work is taking place should be enough.
I don't know what your ground is like, so this may not be a problem. However when draining fiberglass pools, I have seen fiberglass builders, brace the pool. Inside, so that the ground doesn't push the pool in, from the outside.
Unless the niche is cracked, it should be repairable from inside the pool. New gaskets and some silicone in the conduit should stop the leak, or at least slow it to a trickle.
How much water are you losing anyway? Is the ground behind the niche staying wet all the time?
Anyway, good luck with it.

The light installation on fiberglass they cut a hole and outside the diameter of the circle they drill holes for the screws to hold the niche and light ring in place with another ring that is on the backside of the fiberglass so its possiable to do from the inside if the niche doesn't have stress on the conduit and it springs way out of place when you take out the screws. I think its worth an attemp before cutting the concrete. If the niche springs out you may need a helper to try and move the niche while you screw in the light ring. You might have to get some long temporary screws to get the niche in place then swap them out with the stainless ones.
Hope that helps.