1. Too much heavy weight training.
Surfers are just as agile as they are strong, heavy weights can kill your speed and add unnecessary bulk. Strength training is an important part of surf training but keep it bodyweight based (like a gymnast) to develop a good strength to weight ratio.
2. Non Surf Specific Exercises.
Perfect technique lunges, squats and pushups will not help your surfing style. If you are a complete beginner to strength training then it is good to learn perfect form, but once you have ticked that box its time to move on. Keep your training 3D and functional to prepare your body for all the awkward positions surfing demands.
3. Just Get It Done!
This is a big part of the gym culture and does not help your surfing. Standard gym exercise programs and trainers often insist that you push your limits and grind out your reps – this works great for muscle toning and weight loss. But as surfers we must train for body control, efficiency and grace – like a dancer or martial artist trains.
Summary – Choose functional and surf specific bodyweight exercises and work on increasing efficiency and grace as well as strength.