If it's the outdoor type spotlight I think you're referring to, I suspect the LED inside is simply a 30 watt multi-chip array similar to the 10 watt multi-chips I used for my DIY LED build, but a little larger. Here's a photo of the 10 watters.
The 30 watt multi-chips are available (without the housing of course) for less than $10 on eBay. All you would need is a driver & a housing after that for a DIY solution that wouldn't even cost half of the one linked to at the Home Depot and you could order the colour(s) more suited to a marine tank. If you're not into DIY, then I reckon the ready made spotlight is not a bad option for the price. The housing will be a bit heavy & clunky though.