I don't think swimmers actually use either term, at least not when talking with each other. UNLESS it is using "lap" in the phrases "I got lapped," or "I'm putting the orange card in, she's on the last lap of the 500." In that second scenario, the swimmer is IN HER LAST 50, since the counter is put in while the swimmer approaches the last 25 or 50 (lc) turn finishing the race.
In terms of individual lengths of the pool, swimmers almost always use "25" or "50," depending on the race course. I can't think of any swimmer or coach using lengths or laps in any set, because it's always something like "400 warm-up," or "8 x 25 sprints."
The only time I use "laps" is if I tell someone who doesn't know that much about swimming that I'm going to the pool. I might say "I'm going to swim some laps," and after, mention the distance swam (in yards or meters; never laps, lengths, km, or miles).
In terms of individual lengths of the pool, swimmers almost always use "25" or "50," depending on the race course. I can't think of any swimmer or coach using lengths or laps in any set, because it's always something like "400 warm-up," or "8 x 25 sprints."
The only time I use "laps" is if I tell someone who doesn't know that much about swimming that I'm going to the pool. I might say "I'm going to swim some laps," and after, mention the distance swam (in yards or meters; never laps, lengths, km, or miles).