Chamber maids are so often overlooked. We think they are among the most important people to tip. Visa and MasterCard are widely accepted. AMEX and Diners Club are accepted in select locations. When using your credit card you are charged with the official exchange rate in Costa Rican colones.
Some banks have automatic cashiers for money in cash.
However fit and healthy you are, do take out medical insurance, preferably one with provisions for flying you home in an air ambulance in the event of an emergency. Here in Costa Rica we have excellent medical care and if you were to get sick or have some kind of an accident while traveling. Nevertheless, while much more reasonable than in the US, medicine and treatment here can be expensive therefore we highly recommend insurance. No it is not necessary to bring water from your home. Water is usually safe in San Jose and the highlands.