We just returned from 3 weeks in CR, the last 5 days in Uvita. The highlight for us in Uvita was the full day snorkeling trip (with Bahia Adventures) from Uvita to Canu Island and then on to Corcovado National park for picnic lunch and hike to waterfall. I snorkeled with a lovely sea turtle! Great clarity of water and variety of fish. Saw dolphins too.
We actually never went to Marina Ballena, but loved Playa Ventanas (the one with the caves - go at absolute low tide) and Playa Hermosa (lots of romantic shady spots).
We liked the food and casual atmosphere at La Fogata. Pizza one night, roasted and BBQ chicken the next. Not fancy but actually really good. Sangria.