I think the biggest problem is strobe placement. I agree with Mike that with one strobe setup, the best position generally is right above the camera so you can get even lighting.
The diver picture, only the eel and foreground get lit up. It would be nice if the diver get a bit more illumination as well.
Second picture, the strobe was not aim properly only half the field was lit up.
The easiest way is just to have the strobe center above the camera initially, at least you know you will get even lighting then move the strobe around if you want to experiment with shadow.
Wide angle shot is a lot tricker to light and I don't always do it right either. 2 strobes certainly will give you wider area of coverage and a bit more forgiving at aiming!
I assume you have a diffuser on, if not, I would keep it on especially for wide angle as will give you a bit more angle of coverage.