When it comes to water perhaps no country reaches the irony level of Costa Rica. It is recognized as one of the ‘greenest’ (both literally and figuratively) countries on the planets. It is the only country in Central America where you can drink tap water without fear of spending the rest of your trip worshiping a porcelain God. And yet in spite of this the most polluted river–the Tarcoles River–in Central America is in Costa Rica. It carries the waste water of nearly 50% of the countries population into the Gulf of Nicoya.
Now that said, unless you should choose to drink from that river or in a few specific very rural areas, you will be 100% fine drinking the tap water in Costa Rica. I say this as someone who is very sensitive to these things. Despite my best efforts at being careful I’ve gotten sick in two out of the other three Central American countries I’ve visited from bad water (it’s always brushing my teeth that gets me!). In years living in Costa Rica, literally drinking EVERYWHERE I’ve never gotten sick from the water.
So don’t buy a ton of plastic bottles, they’re costly and bad for the environment. Get one or two bottles and just keep filling them up. Ran out of water and not near your place of lodging? Ask someone–seriously. Just go up to a house and they will fill your bottle up for you, if you look hot you might even get some cold water if they have it. Costa Rican’s are very hospitable and always willing to help.
Plus remember that in the tropics you sweat a lot. I personally had to drink about 1.5 gallons a day when I first moved here to make up for all the sweat loss. As one survivalist put it “In the jungle, if you don’t feel like you need to pee, you’re probably dehydrated”. So keep hydrated, it’ll make your trip more enjoyable and you’ll be healthier to boot.