Fitness for diving includes cardiovascular fitness to maximize respiratory efficiency and minimize oxygen consumption, flexibility to reach your own valves, and strength to lift your scuba equipment and fin efficiently. Dive research suggests that poor fitness is one contributing factor behind many dive injuries and drowning. It is beyond the scope of this website to address all aspects of diving fitness, but we will look at the "core".
One of the most strenuous portions of a dive occurs before you even enter the water. Full dive gear can weight up to 30 kg out of water so it's important to strengthen the muscles required to lift this load from the dive centre to the boat, on a rocky boat, and up a narrow ladder.
Once in the water you are weightless. Neither feet nor hands are in contact with a stable surface so all propulsive forces originate from a stable trunk. Finning takes more strength than people realize and even with a lot of strength training behind you it's difficult to train for that specific environment. Finning uses the front and back of your legs, so you also need balanced leg strength, which is difficult to achieve with land exercises. Without proper preparation, you will fatigue quickly, especially if you're finning upstream