If you research it enough, I think you may find that it's pretty serious business to try to get the same illumination from an LED lamp that you can get from that 200 W halogen lamp. You not only need a lot of light, but you also need it going to the right place, i.e., delivered to the optic fiber.

As efficient as they are compared to incandescent lamps, LEDs still produce some heat in operation. It may not seem to be that big of a deal when one looks at a small LED flashlight, but we're talking about a lot more light here - and more heat that has to be dissipated.

Why don't you check with the folks who currently sell fiber optic pool illumination systems, and find out what they offer in the way of light sources for those installations? If it's straightforward and practical to do it with an LED source, then they might be doing so. If they aren't doing it that way - if they're only using halogen or high intensity discharge lights - then the reason(s) for that might be of interest to you as you try to decide how to proceed with your system.