kupati

1. (kupati se) to have a bath; to bathe
   Uveče se uvek kupam pre spavanja. — I always have a bath before bed in the evening.
2. (kupati se, sea, river) to swim; to go swimming; to bathe
   Na letovanju smo se kupali svaki dan. — On holiday we went swimming every day.
   Da li se ovde sme kupati? — Is swimming allowed here?
3. (+ acc.) to bathe; to give a bath
   Tata svako veče kupa bebu. — Dad gives the baby a bath every evening.

The perfective is okupati (se); kupati se in the sea can mean bathing or playing in the water without actually swimming (plivati).