Do in ground pool lights use conduit or just the wires buried deep below the surface? I have a side view drawing of the pool when constructed but it doesn't show electrical wires ,just a side view of the pool.
The code requires conduit, either red brass (not used for decades) or Rigid Nonmetallic Conduit (gray PVC) There would be no way to replace or even install a light in a wet niche without it. Typically there is an unspliced STOW cord going from the sealed light assembly to the elevated junction box somewhere near the edge of the pool deck through this flooded conduit. There will also be an insulated #8 bonding wire in there that bonds the niche to the equipotential pool grid. (all metal within 5' of the water) The connection to the niche is "potted" in epoxy or other listed compound.
I doubt you will have any luck finding a LED replacement for a 300w halogen T3 RSC bulb. That seems like quite an odd duck for a for a pool light.
If I were in your position, I would identify the Light fixture currently in your pool and see if I could find, on eBay or Amazon, a replacement fixture that would fit your existing niche and that would accept an LED bulb. I just can't imaging pissing away that much power for a single pool illumination light bulb.
Depending upon your current light niche, I am finding quite a few fixtures for $100 - $150. I am now going to bow out and leave any further detective work up to you, good luck.
I happened across this interesting looking product from one of the big names in the pool business. I have no idea what it would cost you from a discount house.
The conduit goes directly into the wet niche and the cord to the lamp runs through the flooded conduit. That is why the junction box has to be elevated above the water line. There is pool water in the conduit.
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