An old, non-swimmer prior to joining the Navy chiming in. I went through AOCS in '76 and was on "sub-swim", as it was called then, almost my whole time in P-cola. The sub-surface survival exercises were a killer for me. Just couldn't relax enough or hold my breath long enough. Twice a week I had to march to the swim tank after evening chow for remedial swim classes. I did master the treading water part and learned how to relax and did OK with that, though. The Dilbert Dunker was actually fun for me and I did enjoy that and the "slide for life" out in the bay. The helo dunker didn't come on the scene til late in my career and I actually escaped ever having to do it, due to it being inop when my swim qual was due for renewal.
Back to P-cola and AOCS. Candidate "J" and myself were the only two in our class struggling with swim. One day, just before evening chow time, our names rang out over the intercom to report to our DI's office immediately. We ran down, performed our office entry, and proceeded to stand at attention in front of his desk. He then proceeded to explain he wanted to help us since we were having difficulty mastering some of the swim requirements. We relaxed a bit and started thinking he might have a heart after all. He asked if we had to do a stroke that incorporated a frog kick. When we responded yes, he immediately shouted, "Frog f***ers begin!!!" As we went to the floor and began performing burpees, he circled us, lecturing us about not trying hard enough, being lazy, etc. After several minutes, the lecture stopped. Afraid to piss him off, we continued our exercises til we lay in our own pools of sweat in the middle of his office wood floor, exhausted. Expecting to hear his voice resume the lecture, we lay there panting, looking at each other. Silence. We slowly sat up and looked around. Nobody around. We rose to our feet and peaked out the door and down the passageway. Nobody in sight. We went back across to the other side of his office and looked out the window to where his car was always parked. The space was empty! He went home that evening, most likely laughing all the way at his subjects PT-ing in his empty office. We never heard another word about that, and we both passed several swim tests on the last day and graduated with our class, so I guess his instructional technique was successful.