(ちんぼつ) Something sinks. Usually a ship. Used with accidents or disasters.
(しずむ) To submerge (a rubber ducky in the tub, a submarine submerges). Doesn’t always have the bad meaning of 沈没, but it can: Venice is sinking into the sea 1 cm a year!!! Also, unlike 沈没、沈む can be used figuratively:one can become ‘submerged’ in grief, self-pity, depression.   $$$TTT
(もぐる) To go underground / dive underwater – literally or figuratively:used about fugitives as well as scared flounders.