Jaco is the first developed beach town on the pacific coast when going south from San Jose. Its proximity to San Jose makes it quite popular and relatively crowded in Costa-Rican terms.
It is also possible to swim but ask about rip currents first. If you are more interested in a quiet beach than parties and company you would be better off staying at one of the small villages outside Jaco like Playa Hermosa, which offers a much more tranquil atmosphere with higher and stronger waves.
Playa Hermosa Jaco
Located 10 minutes drive south of Jaco, Playa Hermosa is a surfers paradise. The waves at Playa Hermosa are stronger and higher than these in Jaco, and it is not a good place for beginner surfers.
The beach is very long and continues south from the village, towards the point of the great tree, until the beach of Tulin, which is the only beach around with decent low tide waves. The rest is mainly high tide.
If the break on Playa Hermosa is too rough for you, try Playa Esterillos, 20 minutes drive further south of Hermosa, as the waves there are the same size but are more swell like, and break much more softly.
Please note that there are about 3 different beaches in Costa Rica which are named Hermosa, but the one which is south of Jaco is the most popular.