Thanks for the advice Bill, my other thought with a 'zoom' lens' would be keeping it stable to get a sharp image especially it is was deep and darker.
I have heard only good things about the Tokina 10-17 but they are pricey !
Quick final question, with shots of subjects which are further away are strobes still effective ? (at the 2m+ range) or is it better to try and work with the natural light ? I realise this would be dependant on strobe, water conditions etc !