If you use a small aperture and a low ISO, with an external flash, then it's quite possible that when you are under the water, a slow shutter speed won't cause the problems with image blur due to camera shake or subject movement that you are worried about. There might be so little ambient light in your exposure that all you have to expose your images is the flash pulse. I don't know how fast the flash pulse is with your flashes but with most "land use" flashes it is less than 1/1000 of a second.
The way to test this is to leave the camera set to "FLASH ON". But turn the switch on your flashes to OFF. And then take a photo. If all you get is a black screen, then you know that the ambient light is really underexposed. If you set the ISO to 80 and the aperture to F8, you'll probably find that this is exactly what will happen under most light conditions that divers encounter (unless you are in clear, shallow water on a bright sunny day). If that happens, then even though your shutter speed is 1/60, the ambient light is not getting through to your camera. So the 1/60 is only giving you a black screen and it won't make any difference whether that black screen has camera shake or not. Then you can turn your flashes back on and rejoice in the fact that the "real" duration of the exposure that is going to create your image is only the duration of your flash pulse - which is probably 1/1000 or less. And that won't cause camera shake blur - and it will probably freeze your subject even if it is moving a bit.
This changes a bit if you use a focusing light. So if you haven't fallen asleep from this long post and are still interested, then let me know and I'll tell you how a focussing light would affect this.