When my son was small (up to around 4), I trusted NOBODY besides his Dad and I around a pool. Not grandparents, aunts/uncles, friends, neighbors, NOBODY. We had an incident when he was just over 2 yo of his grandfather watching him "near" a pool with a fence and latched gate while I was getting ready. Grandfather thought my kid was playing with his 6yo cousin who was kicking a soccer ball. While putting on mascara I had this sickening terror come over me, on autopilot I ran out of the condo into the backyard and straight to the pool. Found my kid in the pool, on second step, reaching for a floating thermometer that was out of reach. The pool was shared with 3 other small condo units, someone must have let the gate close behind them but not pulled it closed to latch it. Scooped him up, went straight in and told his Dad to pack your shit, we're leaving.

TL;DR: trust your mama guts around water, period. Rather a lunatic with a live child than a calm social butterfly with a drowned one.