I got to watch some pre scuba recruits get smoked at the Ft Campbell pool. For 2 hours, the recruits were forced to do various exercises in the water. Bobbing up and down for 10 minutes, treading water, swimming circles around the pool with their hands behind their back and many other fun stuff. Near the end of it recruits would hit muscle failure and go down. There were instructors in the pool to pull the recruit up and to the side of the pool. Once a new exercise was started, the recruit went back to swimming. Remember, all this was done with 10 and 20 pound weight belts.
00b (scuba) can be attained through the bear program if you are on active duty. I looked into going 00b. Here is a web site that can explain it better than I can
You'll be an engineering diver and assigned to a unit within the Corps of Engineers. Salvage, construction, maybe some demo .. basic underwater engineering duties. Many confuse it with combatant diving, but they are two different schools.
You'll need to be a strong swimmer, as well as be in excellent physical shape.