Local surfers can be dangerous to those who are new to their territory. They protect their waters in the same way homeowners protect their homes. They feel a sense of ownership. Any stranger on their waves implies a trespass. If you respect the local surfers, they may have no problem with your sharing their waves. If you show them a lack of respect (by dropping in, cutting them off, etc.), it may be safer for you to find other waters in which to surf. Don't take the locals lightly. In some areas, they've physically assaulted new surfers to protect their waters.