My suggestions...
Like someone said be4, nothing will make you a better uw-finner than finning uw...
Next best thing, IMO, is to go swimming. I do about 2k, switching between freestyle and breaststroke every lap, it takes me less than an hour (incl changes) and for me this is a decent workout. Just make sure that you don´t use your arms much (if your goal is to get better at diving). If swimming isn´t your thing then I´d suggest cycling or running. All of these or any other endurance workout should help keep you comfy in water (they do for me anyways). I play football and any other sports like badminton, squash and basketball should help you as well as they consist of a lot of starts and stops which should help you with muscle strenght. I´m not a gym-person so I´ll leave the weightlifting advice to those who actually do any of it.
Do something you enjoy (or at least tolerate) and you´ll be able to keep doing it, regardless of what it is, is my advice....
Like someone said be4, nothing will make you a better uw-finner than finning uw...
Next best thing, IMO, is to go swimming. I do about 2k, switching between freestyle and breaststroke every lap, it takes me less than an hour (incl changes) and for me this is a decent workout. Just make sure that you don´t use your arms much (if your goal is to get better at diving). If swimming isn´t your thing then I´d suggest cycling or running. All of these or any other endurance workout should help keep you comfy in water (they do for me anyways). I play football and any other sports like badminton, squash and basketball should help you as well as they consist of a lot of starts and stops which should help you with muscle strenght. I´m not a gym-person so I´ll leave the weightlifting advice to those who actually do any of it.
Do something you enjoy (or at least tolerate) and you´ll be able to keep doing it, regardless of what it is, is my advice....
The mt biking/cycling might work well too 
