Lower is better don't even think about failure temperature , you should keep your led (and that mean emiter core - not star PCB) on temperature cca 20°C over ambient temperature ! Same thing as by computer procesor !
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If you mean to keep the die temperature at 20C above ambient, you'll have to run the emitter well below it's maximum ratings. The SST-90, XM-L, XP-E, and XP-G all have 20C or more
just inside the emitter when run at full power. Of them all, the XP-G is the best, with a nominal temp rise that has to be rounded up to get to 20C (calculates at 19.5C).
Unfortunately, a more reasoned temperature analysis requires an understanding of thermodynamics that's far beyond explaining in a post. But to answer your question, the remainder of Lucca's response is pretty reasonable. You want to know how hot the copper slug will get. If you have lots of contact area between the copper and aluminum, and lots of contact between the aluminum and the water, the copper won't be much warmer than the water. In air it's a whole different story, but since this is a dive light, we don't have to go there.
It remains only to define what constitutes 'a lot of contact area'. At under 30 watts for the SST-90 (yes, the total power input is probably more like 35 W, but some is put out as light), 1-2 square inches is probably sufficient to keep temperature rise within reasonable limits, assuming tight fits and full contact with good thermal compound.
Wait a minute!!!! I just looked up the CTE of aluminum and copper, and aluminum is 50% higher than copper. That means that as things get hot, your gap will
expand, not contract. You should worry more about loosing contact than anything else. On the other hand, if you lose contact, the copper will get hot and expand, while the aluminum won't. Sounds like a self-regulating system. I think Lucca's answer is about as good as you are going to get.
I'm not sure about the 0,006mm clearance. That's beyond most hobbyists' ability to machine or to measure. If you can achieve that, I'm impressed. I'd hope that 0.05 mm is close enough