You can't go wrong with a DS160. It'll upgrade well down the road and it puts out some serious light. Money well spent. Photography is, afterall, all about lighting. You can verify all this good advice by turning your strobe off for a shot or two and shooting natural light. When you shoot natural light the filter is best but you might try this at depth and just see what happens.
1. Set ISO to 400
2. Manual white balance off of the sand on at depth
3. Make sure you're storing in RAW.
Take a few shots to experiment. Never hurts because you never know when you're going to be without a strobe for any number of reasons.