Chile's beaches, need your help

Depends what you like.
Vina is a urban beach, and is usually crowded over the summer, especially during the Festival. Beach in Vina itself is not the nicest one, Renaca (a few kms North) is a bit better.
IMO, San Antonio is similar, less glamourous (well, less than Vina and Renaca sector 5), but not the nicest beach and pretty crowded.

Maitencillo has a nicer beach and a few things to do.
Quintay has a cute beach (not white sand if I remember well) and is still a small fishers' village. Accomodation can be pretty basic.
Quintero offers a series of secluded beach (white sand), but I have no idea if it's crowded in summer, as I always go there in lower season.
Isla Negra could be nice as well. Small beaches with black rocks, nice enough to have inspired P. Neruda. A couple of decent hotels and restaurants around.
Zapallar has a nice small beach, and it's a quiet spot especially cherished by upper class Chileans (thus, is more expensive than other beaches).

IMO, though, the nicest beach in the 5th region is Salinas de Pullally, but you can't stay there overnight (no accomodations and camping is not allowed, yes, there's even a municipal rule that prohibits beach camping) and you'll need a car to get there.

RE: San Antonio area. San Antonio is a working port not a beach. But do stop at the information center in town for information on the beaches and other attractions to the north and south. Do check out the rustic fish market to get a glimpse of Chilean sea life and the work of the pescadores artesenales. Do walk along the fixed up waterfront with their kiosks of Chilean arteseanal gifts and knickknacks and do eat at one of the restaurants near the fish market for ultra cheap and fresh delicious seafood.

There are beaches north of San Antonio (listed in order): Cartegena (very crowded this time of year - maybe this is where #1 was talking of, considered a "lower class" resort town, do watch your stuff), Las Cruces (known for culture and art), El Tabo (known for it's natural areas and wildlife), Isla Negra (#1 covered this area, definitely do the tour of Pablo Neruda's house if you make it there), El Quisco (resort town and nearby rural historical area), and Algarobbo (nice resort town but more expensive).

Just south of San Antonio is the exclusive beach area (sand not bright white though) known as "Rocas de Santo Domingo" where many rich Santiaguinos have huge second homes with beautiful gardens. Nice beach but definitely no overnight, no restaurants/cafes/lodging unless you go further into the municipalidad or rent a condo. There are music concerts at night and other artistic events during the summer.

If there are no direct busses to some of these towns, you just stop at the closest town to your destination and take a collectivo (fixed route taxi) or two to your ultimate destination. At least basic Spanish is a must to get around anywhere in Chile.

vina del mar is very touristical and crowded in the chilean summer. mainly populated by argentinians.
san anthonio is a port, not a beach town. note that the chilean sea is cold because of the humbolt stream, no matter where you go, even in arica.
henny

Actually the water this year is noticeably slightly warmer to warmer (but still cold) due to El Niño.