Training with heavy weights in the gym will hinder your surfing. Body building exercises like squats, bench press will make you stronger, but they will make you a stronger weightlifter. As you know athletes do a lot of heavy weight training to enhance performance, and for contact sports and power based athletics it is essential. Surfing is different, surfing is a flow sport, we want to be strong in terms of dealing with our own bodyweight (like a gymnast) and we want to be able to move freely and smoothly though all sorts of positions, whilst maintaining balance and poise (like in dancing and yoga). If surf fitness is more important than a big guns then stay away from heavy lifting in the gym.