El Nino Watch
The national level describes the worst province in Thailand; the Samui level describes the island. A national flood mostly affects Samui through travel and supplies, not directly.
Koh Samui watch
Koh Samui (Maenam) is about 470 km south of Bangkok and about 390-410 km from Pattaya and Sri Racha, across the Gulf of Thailand. Heavy rain there does not flood the island.
What it can do:
The distance note on the Thailand page (under "What this means for Samui") is written from the live data. The rings on the map mark 100, 250, 500 and 750 km from Maenam.
Keep two answers apart: "is the island in danger?" (Koh Samui page) and "can I get off the island, or get supplies, if I need to?" (this page and the Exit plan).