Assuming that you haven't got a nice helpful manufacturer like above:
I made the same comment recently in a camera thread. What does the damage is the energy from the batteries causing electrolysis.
If you see your light flooding at depth, open it immediately and get the batteries out.
When you get back on the boat rinse the light with fresh water and then keep it immersed in a container or plastic bag of fresh water until you get home or to a workshop. (I'm sure everyone remembers the science experiment from school that leaving a piece of iron in air or in water doesn't cause rusting, but leaving it wet in air does.
)
Rinse and soak with distilled water - then dry with a blow dryer or hair dryer (without excessively heating any electronic components or modules). Then leave for some time in a bowl of rice or something else that will absorb any moisture coming out of circuit boards.
The distilled water won't leave any residues that could alter circuit performance.
You may then have to clean up some contacts but they should not have corroded away completely.
I've saved more than one light by following this procedure. (And even the same LED light that repeatedly flooded until I got the required o-ring size exactly right).
There is also another advantage to this procedure - the risk of the light exploding is diminished. My son had a well known brand of light flood at 40m. He simply stuck it in his BC pocket and pulled out his spare. While he was on the deco stop there was an enormous "thump" and he felt like he was being struck hard (it left a bruise on his rib-cage). It was the dive light exploding in the BC pocket due to the pressure build up from electrolysis combined with the external pressure reduction.