roniti

1. to dive; to swim underwater
   Leti volim da ronim u Crnoj Gori. — In summer I like diving in Montenegro.
   Deca su ronila tražeći školjke. — The kids were diving for shells.
2. formal (of tears) to shed
   Majka je tiho ronila suze. — The mother was quietly shedding tears.

The perfective is zaroniti (to dive in once).