There is not really snorkeling in Costa Rica, a few spots, but the Pacific side is more for surf. The caribe side below Pt Viejo is your best bet. On the Pacific side, Playa Bellena (low tide) at the whales tale.
There is not any white sand beaches and crystal blue waters in Costa Rica either, its mainly brown sand (volcanic) and green waters, due to color reflection of the sand, in the water. The lightest colors of sand near white, are mainly on the caribe side as well, Cahuita, Playa Cochles, Chequita, Puna Uva and Manzanillo-Ganduca. On the Pacific side, play Conchal near Brasilto and Tamarindo is nice too...but its a small beach.
Playa Samara, like all the Pacific beaches, will be crowded or booked Xmas week, its a huge holiday for tourism, and also locals that week. Xmas is a tad peaceful for locals, but the NYE weekend will be jammed, they camp on the beaches everywhere. The traffic is so bad they reverse the highway toll to all one direction back to San Jose from Puntarenas.
The Pacific side is most developed and caters to tourism and beach holidays, so at Xmas/NYE, I would avoid it. Manual Antonio is best avoided year round. Its a small park and small beach, and the area is very developed, nothing Backpacker about it, its a destination for honeymooners and families/couples, and is jammed that week. (Closed Mondays). Why in the world would you even consider Manual Antonio, it hardly has a beach to say least.
The best area for beaches and nice sand, few crowds and fewer tourist, and lower prices, is the caribe side. Rainy season is afternoon showers if that in end of Dec, the weather in Dec in the country is great overall...but the Pacific side will be brown and bone dry, dusty roads and not a lot of green, from Papagayo to Tamarindo to Nosora, Samara and Montezuma and Mal Pais. The southern zone, south of Quepos, is greener due to rainfall almost year round, as is the caribe side, so another reason to head to caribe.
Lastly, if you want a backpacker vibe on the Pacific side, then head to Mal Pais and Santa Teresa, its the end of the road, and only those in the know, go down there, and great surfing too.
But Pt Viejo and the cairbe coast is far more chill out backpacker vibe, and also more authentic and rustic, than anything on the Pacific side, period.
But, if you head down to Playa Pavones, you will have nothing to contend with but great surf.
What guidebook are you using, with so much time in the country, you should have a good one, like Moon, Roughguide or LP....
Your info and impression of where to go not very accurate.
http://nicoyapeninsula.com
http://www.puertoviejosatellite.com