Tom is right. Normal Sync Speed (shutter window is fully open) is determined by the camera, not the strobe. Underwater strobes work only with fully open camera shutter, because they produce flash mono pulse only. Underwater strobes don't support HSS.
Mechanical shutter of modern DSLR cameras has sync speed about 1/200 s. This is fast enough for 99% of underwater shooting tasks. Don't forget that flash duration is just 1/1000 by itself, it freezes moving underwater objects very well.
If you need more fast sync speed with underwater strobes for any special aim, then use cameras equipped with electronic-mechanical shutter. For example, one of my cameras Nikon D50 has sync speed up to 1/500. But believe me, for normal cases this is not necessary. I used sync speed 1/500 with underwater strobes only 1 time for all years of underwater shooting. Don't forget that camera has other controls as well, for getting desired exposure.
If you need a speed more fast than 1/500, than use land strobe in the housing. This case High Speed Synchronization (HSS =40 khz flash flicker) is available, for speeds up to 1/8000.