The most important rule in the water: whoever is closest to the breaking part of the wave has right of way.
The base rule is simple: the surfer sitting closest to the peak — where the wave breaks first — has priority, and everyone else lets them go. If the wave breaks both ways, you split it: one left, one right. Someone already up and riding has priority over someone still paddling. And if you miss a wave, you paddle back through the whitewater, never across someone's line. None of this is written down officially, yet it's enforced at every spot on earth — breaking it is called dropping in and is the fastest way to make enemies. Beginner tip: on crowded spots you'll do better taking the second or third wave of a set rather than the first.