I am a bit unclear here. Do you think the swim problem was a pacing issue or a physical issue? I read it as a pacing issue Have you tried going to a pool and just swimming lengths as slow as you can to see how you go? Have you tried other strokes? I know I have never mastered the Freestyle couldn't go a full lap but I can go forever on the sidestroke and backstroke/breastroke. Honestly with my back inflate most of my surface swimming on scuba is nice leisurely on my back chatting with my dive buddies
Swimming normally, without mfs was a pacing issue, I think. With mfs I kept slowly sinking, until the snorkel was underwater, when I got a breath of water. That's not exactly a physical issue, but it is a lack of experience with a snorkel issue. (to wit, my first experience with the snorkel had me grabbing the mouthpiece with my front teeth -- I didn't realize the bite pads were where you were supposed to hold it, or even what they were) With more practice I feel I could do the mfs swim without trouble, but that would have held everyone in the class up while I figured it out. It could also be a poor technique issue, where I'm wasting too much energy on something I shouldn't be (with the normal swim). I do not feel that this is a physical fitness issue -- I made it through 10 minutes of treading water without reverting to floating.
When I try the swim portion again, I'm going to go with a non-crawl stroke. Probably side stroke.