We have found that the best way to label goggles is with waterproof medical tape. Cut off a small piece, fold over strap, and write name or initials with a fine point sharpie. Make sure they are adjusted and fit well before you place the tape.
Electrician's tape is perfect for fins, snorkels, and paddles. Find tape that fits your personality! It is available at hardware stores and craft stores.
Permanent markers are best for buoys.