氵 (water radical) + 召 (to call/summon) - water that summons you in, i.e. a boggy pond. The character almost always appears in compounds: 沼泽 (marsh), 沼气 (methane/marsh gas).
Left three-drops water radical - the indexing radical. Marks 沼 as standing water rather than flowing: a shallow pool, marsh, or pond. Sits in the water family with 池 (pond), 湖 (lake), 潭 (deep pool), but specifically for boggy / reed-edged water.
Right side 召 supplies the sound - zhào with a tone shift to zhǎo. Same phonetic family includes 招 (beckon), 昭 (bright), 照 (illuminate). The 'calling' meaning is purely phonetic baggage here; 沼 stays squarely about water that has gathered and stayed put.
