洲

Left three-drops water radical - the indexing radical. Marks 洲 as a water-related landform: islets in a river, then by extension a continent (chunk of land surrounded by ocean). Anchors it in the water family alongside 河, 湖, 海, 港.

Right side 州 supplies the sound directly (zhōu → zhōu, exact match) and reinforces meaning: 州 originally pictured land masses rising from flowing water - three dots between three strokes. 洲 is the explicit water-marked version of the same word; over time 州 specialized for administrative regions and 洲 for the physical islands and continents.