Swimming is by far the best. I usually swim a length at a good speed (crawl only) turn round then dive and swim as far as I can underwater (to increase lung capacity) then when I break the surface, swim again at full speed for a few yards (to simulate coming out of a duckdive and having to paddle over the next wave to get out back) then slow for a bit to the end of the length, then turn round and start again.
I also practice swimming underwater until I absoultely HAVE to come up for air. I can currently go for 2 min 17 secs.
Cycling is good for the legs, pushups are good to practice popping up.
I have a 'custom' squat thrust, which is 'pop to my feet in a surf style' then jump as high as possible to build the legs.
Any cardio exercise is good for building stamina!