The AMC7135, often abbreviated '7135 or just 7135, is a linear constant current driver. They are often used in multiples. This board has 32 of them, each one putting out 380 mA, for a total of around 12A. It does look like it was made for that style light, so it might be wise to measure your board and see if it is in fact 46 mm diameter. If you wanted a lower top end, you could remove some of the chips and have less total output. Note that many 7135s only put out 350 mA, but higher current versions are available. MTN specifically says these are 380 mA.
If someone is making drivers for these lights, they (or someone else) might be making LED boards as well. It's worth checking into. Making your own copper boards isn't hard, but it isn't cheap. The one low-cost overseas place I found that offered copper would do 5 boards (minimum quantity) in fiberglass for $25. In aluminum they were $82, in copper, $400.
As far as heat dissipation, something around half the electrical power delivered to the LEDs will be converted to light, the other half to heat. So I would base calculations on dissipating 15W, not 30W.
Offhand, I'd guess that the biggest temperature drop would be between the MCPCB and the case, but we'd have to have much more detail about the thermal path from the LEDs to the water before reasonable estimates could be made.
...up in Canada the bottom of our lake doesn't get above 42F. Usual temps in summer are 65-72F surface, Sudden thermocline at 30ft to 50F after that it's 42F down to 370ft...but never been down there to test that hypothesis 
