Left water-radical 氵 (the three-dot side form of 水) - the indexing radical. Anchors 深 in the water family alongside 河 (river), 海 (sea), 浅 (shallow). The original referent was deep water - a pool you can't see the bottom of.
Right 罙 supplies the sound shēn exactly. The graph itself depicts a hand reaching into a cave - perfect semantic resonance: probing depths. Phonetic family: 探 (probe), built on the same 'hand-in-cave' element. Together with water on the left: water that you can probe deeply into.
