Looking at that transformer and what you said about the lamp, 300W, that's 25 amps at 12V. Total resistance of your run works out to be .1164 ohms. At 25 amps that's a 2.9V drop. If the lamp is indeed 300W then moving to a higher voltage tap won't cure your problem, just make the lamp brighter. 25 amps is a lot of current and localized heating at the lamp base contacts is probably what's killing the thing. I don't know anything about pool lights and am just making this comment based on knowledge of Ohms Law, and some physical characteristics like conact resistance issues thrown in.