Unless you are going to the beach on Easter Weekend, 90% of Costa Rica's beaches are desolate 90% of the time. Manual Antonio on the other hand, is always busy, as its small, and next one of the most popular National Parks.
Otherwise, there are no crowds at beaches, especially weekdays, few people. The only beaches that can be a tad busy, are Jaco (Best avoided like Manual Antonio too) and possibly Tamarindo on Easter.
Beaches in the Osa, and southern zone, don't even have tourist on them, and locals unless they surf would only be there on weekends.
As for beaches on the caribe side, they are gorgeous, small and rustic, and very desolate overall, as there is limited lodging and not a locals either.
The more important factor is, what beaches can you swim safely at, as this list os very short. Most the Pacific beaches are rough and better for surf. Besides Tamarindo, Samara and Montezuma, and MA, you need to be very careful swimming on the Pacific side.
On the caribe side, most the beaches below Pt Viejo are fine, depending on the month.
The other factor you need to worry about, is Dengue and Zika and Chiga, what month you going, as each coast, and areas, had different tropical micro climates and weather, and all 3 diseases can be a factor, in rainy months.
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