Ko Kham

A small, navy-managed island off Sattahip with white sand and clear water, open only for day trips. It's popular for snorkelling over the reef, with mats and life jackets free and gear rental cheap (around 50 baht). Beach access itself is free; you pay for the boat out from Khao Ma Jor Pier and any equipment. No accommodation, so it empties out by evening.