Nice to hear from after all this time and thanks for the advice. There are quite cheap little 300m housings and lights available from Golem Gear in the US:
I am not sure if they make them in-house or if they’re made by a Czech company. Anyway, I’ve bought two in the past and they were great. No leaks even after being bashed around on the sea floor.
The important question is whether the new GoPro 9 is good enough in low light or if I’ll have to use a shutter speed that is too slow to capture sharks moving past the GoPro?