WordNet sense 1 (fall abruptly):

dump

WordNet sense 2 (drop steeply):

dive, plunk

WordNet sense 3 (thrust or throw into):

immerse

WordNet sense 4 (immerse briefly into a liquid so as to wet, coat, or saturate):

dip, douse, dunk, souse

WordNet sense 5 (engross (oneself) fully):

absorb, engross, engulf, steep, soak up

WordNet sense 6 (begin with vigor):

launch