I forgot to add that even high end athletes might not have a training program that transfers into the water or diving. My girlfriend is a pro figure skater. She is usually on the ice 4 - 5 days each week and in the gym 5 or 6 days per week. When she tried to do a 200 swim for her open water prerequisite testing, she couldn't complete it the first time. She swims crawl well, breaststroke and sidestroke beautifully, and butterfly average. I can't keep up with her in the breaststroke.. But, she hadn't been in the water for months when she tried to swim 200 yards.
In the water with fins on, she is a powerhouse. Many divers have weak leg, ankle and foot muscles. When they flutter kick with fins, the blades tend to move inward toward one another. The diver has to open the legs to keep the blades from touching. In her case, her very strong legs, ankles, and feet are able to maintain fin blade position perfectly and her kicks are easy and strong allowing her to move efficiently.
Standing on one leg really helps strengthen muscles. In her case, she does figure skating , dance and ballet. Others find yoga increases stability. Or, you can keep it simple and just stand on one leg for a few minutes a day. Research as shown a decrease in field sport injuries such as soccer and football injuries when part of the player's conditioning is balance.
My former girlfriend grew up in the water on a sailboat. She works as a stage and production manager, but doesn't follow a routine workout schedule. She's tried P90X, going to the gym, machines, and treadmills on and off. But, she can get in the water after a long absence and swim a 500 easily. When cave diving with me, she could keep a pace that exceeded 50 feet/min. I believe if water taxes you physically that is where you should exercise.
I go for long scuba swims on some days the way people jog. Breathing through a regulator is more demanding (pun intended) than breathing on land. So, using snorkels when training preps us for that. Keep in mind that breathing is key to underwater safety. Breathing is how we control stress and avoid panic.