LED VS Halogen Fittings in Underwater Applications

If you buy LED lamps from a good name-brand manufacturer the LED lamps should last a lot longer than the halogens and run cooler (overall) for an equivalent light output. In either case, some method of removing the heat will be needed.

The halogens will give you a full spectrum of light with a pleasant color temperature of around 3000 kelvin. The LEDs can provide a similar-appearing warm-white color, but the actual spectrum is a mix of blue and yellow to achieve this. The LEDs lamps are also available in a neutral white (4500 kelvin) and cool-white (6500 kelvin), achieved by varying the amount of yellow phosphor mixed with the blue LED emission to create the particular shade of white.

So if you need a full broad-spectrum light, you'll need to use halogens. Otherwise, LEDs will be cheaper in the long run, cooler, and will last longer.

LED's will work off low voltage DC and won't shatter if water contacts the bulb. much safer all the way around.

So long as the fixture is IP68, the rest is a matter of preference. A LED fitting will consume less energy, though that might not be an issue in a water feature, which is basically an aesthetic structure.

Assuming the fixture doesn't fail of course....