Koh Mat Wai Yai is a Mu Ko Chumphon National Park island site used for boat-based reef visits, where the draw is close-up shallow coral, easy in-water exploration, and colorful marine life rather than technical terrain or heavy current. The underwater route is better known for relaxed snorkeling and simple scuba observation around broad coral patches, anemones, and reef fish than for dramatic depth or exposed drift conditions. It is commonly visited as part of Chumphon archipelago day trips, with access depending on park season, boat logistics, and a no-landing setup on the island itself.