I know Master Card and Visa are accepted most places. I'm not sure about American Express.
Last I knew there was still no ATM for tourists in San Pedro.
I've always found it easier to take travelers cheques. I've found them easily accepted and/or easily changed to money. I start off with $500 in US cash and bring the balance in TC. I usually charge my diving and acco on my credit card but very little else. That way I don't windup with too many surprises popping up on my statement, due to evening activities.
There are a number of places for nightlife fun and Fidos is only one of them. Don't worry, you shouldn't have any trouble finding the other places and may even be lucky enough to come across some off the beaten path, that you will enjoy. Just ask around. You'll want to see the Punta Boyz usually playing at Crazy Canuks on Sunday or Monday Night (can't remember just now)and you can also catch them at Shark's Bar on Tuesdays. For some reason, I just always enjoy the show at Crazy Canuks better.