This was my thought as well when I started reading this thread. As a DM you may be responsible to save a life. As harsh as this may sound, if you're marginal on the swim test I don't want you leading my dive.
@American Digger In Europe wasn't marginal. His 800m in fins, mask and snorkel time of 14:30 earned him a score of 4 (max 5 is under 14mins). a 1 (lowest score) is over 21mins. For further context the 400yd surface swim has a max time of 12m (For GUE Fundies, it's a 375yd swim in under 14 min)
Furthermore the candidate needs to score at least 15 points over the 5 water skills tests. which includes a rescue tow (100yds/m) both divers in full kit against the clock (max time 5 mins for a score of 1) While a candidate might be weaker in one test, it's impossible to pass unless you're strong over the majority
The 800yd/m test is a stamina test, you get almost the same allotted time as you do for free swimming 400yds/m (different times for yds or metres) Swimming hard breathing through a snorkel and having your legs tire from constant kicking in fins, is part of the challenge. Stopping or raising your head above the water is a fail
So this candidate isn't marginal in the least - certainly in respect to a lot of divers who whine and complain when they have to use a line to pull against a bit of current because they're so unfit.
Also your comment about leading a dive.
When is someone actually going to swim 800 yds to perform a rescue?. If the rescuer is screaming through the water it'll take them at least 14 mins to get to the victim, at which point they'll be in no fit state to assist - let alone tow a victim another 800 yds back perhaps making rescue breaths for the first 2 mins or so...
Given @American Digger In Europe has a disability and is faster than some 20-30yr olds (at 59) I think he should be applauded - I'd have zero qualms about him being a DM on my boats