船 (boat / ship) + 底 (bottom). 底 reliably means 'bottom / base' across compounds: 海底 (seabed), 鞋底 (shoe sole), 心底 (bottom of one's heart).
culture
船底座 (Chuándǐzuò) is the Chinese name for the constellation Carina ('the keel'), one of the parts of the ancient Argo Navis, the ship of Jason and the Argonauts.
