Photographing underwater has all the usual challenges of photography + the fun fact that it is done under water and you need to make sure your equipment is safe.
It is also pretty hard to position lights under water, so usually you’ll have a set of underwater strobes hanging of the camera housing. Photographer John Coffey shares a pretty clever DIY to make underwear extendable arms for strobes.
The design uses Loc-Line which is probably the best invention in the word right after gaffers tape. Each arm is hardened with a piece of PVC elasti-pipe and connects an Ikelite Underwater hosing to Ikelite Underwater strobes (though it can be used for other systems as well.
Of course if you go with smaller hot shoe strobes you can use the genius optical chain triggering system that JP Danko invented way back.
[DIY How to Build Strobe Arms | John Coffey]