Thank you all for your replies.
It almost goes without saying that testing is the only way to really understand what works for my particular situation.
I'm off to order some 800Z and 400H first - testing both in various conditions.
I'm the sort of person who would 'test' both films under similar lighting conditions (and then devved at the same time) - sadly it's not very practical - being cold and wet doesn't make for the best conditions to change film on a reef!
Again thanks to all for your replies - i'm looking forward to the learning and above all the anticipation and enjoyment this little film camera has brought back to my photography.