Confession: this was taken with an Olympus E-PL2, which I use more-or-less exclusively for UW. With this system, the strobes rely on optical slave triggers that are connected to the housing by fibre-optic cables where they transmit the light from the on-board pop-up flash. Of course, when I went diving with a pair of (trained) dolphins, I neglected to raise the flash while setting up, and since the housing doesn't feature a plunger to press the button, it remained folded away, limiting me to available light.
Luckily, I managed to adapt and come away with one of my favourite UW shots, which I probably wouldn't have taken were it not for a stupid error. Untitled by colin|whittaker, on Flickr