Not an easy question to answer on the lens.
I agree with Stoo that one-size-doesn't-fit-all. Others will disagree but my own experience is that "capture all" lenses are unsatisfying.
So where do you go from there? I think in many ways you need to decide what you like photographing - or what would you ideally like to photograph. Is it the shipwrecks with a wide-angle? Not necessarily easy in UK low-light, turbid conditions. Or do you fancy a go, or like, doing close-ups of fish portraits. In which case, again, I agree with Stoo - go with the 60mm.
Once you have decided that you can then think about the strobe to go with it. You can do a lot with one strobe if you recognise the limitations. Maybe easier with macro and one strobe than wide-angle. But wide-angle is doable.