It's not a requirement to do the treading water directly after the swim. Your instructor could have given you time to rest or they even could have done swimming and treading water on different days....
It would have been better to do that than to take the snorkel option, imho.
Either way you got through it so that's all that matters.
R..
I disagree, I think it's very important that you do the float/tread immediately following the swim. I think the swim test should serve two purposes: it should show that you are comfortable in the water, and that you are in good enough condition that you have adequate energy reserve/stamina to deal with unexpected events. That's what the distance and tread requirement is for; I can tell if someone's comfortable in the water after they've taken two strokes (and AFAIC, dog-paddling is not a stroke). I expect that was the instructor's rationale in this case - I know the BSA swim requirement is predicated on that.
I'd hope that any instructor would feel the same, although I understand your special circumstances. In California when I was growing up, it seemed as if all kids (except inner-city ones) learned to swim as a matter of routine. We used to have similar certificates (Red Cross?); not sure if they still exist. But if that were the case, I might just ask if someone can swim, what level, and do as you did. However, lacking some kind of _meaningful_ certificate, some people will lie or just overestimate their own competence. Doing the swim/tread first thing separates that group out, and it's very useful to know who's lacks good self-evaluation skills.
I believe a time requirement demonstrates how good a swimmer you are as well as your conditioning, and I firmly believe that as you move into more advanced areas of diving the swim requirement should also become more stringent. For example, GUE's requirement for Rec/Fundies is 300 yds./14 minutes and a 15m/16yd. breath hold. I think 300 yards in fourteen minutes is overly generous; I recently tried to swim breast or sidestroke slowly, and was unable to get my time UP to 10 minutes. Ten or at most twelve minutes would seem a better standard to me. For Tech 1 that goes up to 400yd./14 min., 18m/20yd. breath hold. For Tech 2/Cave 2 and above it's 500 yd./14 min., 18m/20yd. breath hold; I could see bumping the latter to 23m/25 yds or even 30m/100 feet, and I like the NAUI basic swim requirement described earlier, which equals/exceeds the GUE tech standards. But I know that will never happen with the for-profit OW agencies.
Guy