How do I find a good trainer?

The first step is to make sure any potential trainer has a good educational background in an academically relevant area (i.e. exercise physiology).  Furthermore your trainer should have credentials to back him/her up (i.e., a licensed athletic trainer, a certified strength and conditioning specialist, or a physical therapist).  Finally since your trainer will be working with you to improve your surfing, make sure that they regularly consult with your current surf coach, or personally watch you surf.  This is the only way they will be able to provide a program that will appropriately impact your performance!