How do you keep from running over paddlers and what's the etiquette about who gets on a wave first?
Whoever is deepest on the wave, in other words whoever is closed to the breaking part of the wave is supposed to get the wave. And whoever is down the line away from the breaking part of the wave is supposed to pull out of the wave. But that, as Captain Barbossa said in Pirates of the Caribbean, is more a guideline than a rule. And at some breaks such as Malibu or Old Man's in San Onofre, people will routinely drop in on you even when a wave is clearly yours. And when that happens you just shrug your shoulders and accept that it is price you pay for surfing at a crowded spot like Malibu or Old Man's.
As for people paddling out, they are supposed to try and avoid you if you are riding wave. That generally means paddling to the broken side of the wave so that whoever is on the wave doesn't have to maneuver to avoid you. But again, that is more a guideline than a hard and fast rule so you will often find yourself on wave having to maneuver around people paddling out.