get ready....this is going to get expensive for you 
you would be amazed with what you can do with editing, so i would say just focus on composition in the beginning, the great thing about it is it is fun trial and error, particularly when you start to 'crack it' and with the proper research instruction you can make some pretty big leaps in a short space of time. liveaboards are 100% the place to do it as well. Macro without strobes/lights is going to be very tough, but you need to be closer and then closer and then CLOSER. Personally i think when shooting nudis that you want to be so close that you are worried that you are going to hear it squeak.
ISO is very tough as it isn't built with underwater shooting in mind. light just doesn't behave underwater. As long as you have a decent strobe (even 1) then you can set your iso to 200/400 and just mess around with your f-stops....that's the pain of 'auto' as it will jack your ISO right up and everything is ultra noisy...i would also suggest sticking with small slow moving stuff to begin with so you can take 10+ shots of the same subject at same distance across F-stops and compare them. I found that was the best way to get comfortable with 'oh. that's what happens when i'm that far away with that f-stop'
I would also say focus on liveaboards with some sort of photog component. you are usually only going to pay an extra 2/300 bucks and get a whole week of instruction and discussion/advice. The 2 mikes on the aggressor jim church are very good. mike meskelski would sit up until after midnight helping everyone edit photos and discuss compositon. What a great guy.. that cayman trip last month was my first photog instructional. i honestly didn't even know what iso and f stops were before then 
i should add that i only started underwater photography in late January, (messed with gopro and tried to cut stills for a year beforehand) so i'm not an expert, but these have been my main learning points...
Anyway, here are some of my shots.. https://thefkproject.smugmug.com/Diving-EditedSigned when you click on the link it doesn;t work, so you need to cut and paste.
sorry, i don't know how to embed a link... so you will have to cut and paste the url. or check out my instagram : ministryofgiraffes