Left-side water radical, the three-drop form of 水. Anchors 濑 in the river family - the character names shallow rapids running over a sandy bed, where the water sense is literal. Same indexing radical as 河, 海, 流, 滩.
Right side 赖 supplies the sound - lài stays lài with no shift. The 'rely on' meaning is not transferred; 赖 is a sound tag only. Compound mostly appears in place names and the literary noun 濑 itself, naming swift water over stones.
