I still have some confusion as to what constitutes a lap, which I always thought was down and back, finishing where you started, lap around the track, I just laped you, I swam a lap, but I digress. To me I would swim 15 laps to = 750 meters
Just be 100% sure you are swimming in a meter pool and not a yard pool. If it is truly a 1/2 Olympic pool it will be a meter pool. However, I have talked to people who swam for years in a yard pool, but thought they were swimming in a meter pool. A yard pool would be 750m = 820.21y = 32.8 lengths of the pool.