The waterproof to 5m isn't so it can survive being 5m deep it's a measure of it's resistance to dynamic pressure.
If you drop it from perhaps 0.5m into the surface, or it gets hit by a wave - it's going to feel an instantaneous pressure equal to being >5m under water.
That's why your watch says waterproof to 150m, it's not so you can break the freediving record it's so you can wear it while splashing your hand hard into the water when swimming.