The beach cambio hours for me have been hit or miss... I follow the following guidelines: go on weekdays & go between 11am and 3pm. This has always worked for me!
You can also call before you go to make sure they are open. Pretty sure the number is 876.957.4974.
Not only hit or miss, but sometimes they have little money on hand. I have a couple of times tried to change 200-300US and they dont have it so you have to go back the next day.