Most restaurants and hotels have purified water so there should be nothing to worry about. If you are concerned, there's plenty of places to buy bottled water in Cabo. But seriously, our family has been going for years and never had a problem with the water in Cabo.
Having to drink bottled water in Cabo is a common misconception. You should be fine drinking most water, other then sea water of course. Because that would be silly.
You should be fine with all the water that you find in Cabo, other then the sea water of course! :P
But seriously, I wouldn't worry about the drinking water.
Something to think about: You will probably have to specify if you want tap water at restaurants as well. Getting bottled water everywhere can get expensive.
My only word of advice is if the restaurant is off the beaten path, I would stick to bottled water. I have also seen street vendors with fruit flavored water that might not be the purest. Other than that, you should be okay. Oh yeah, sometimes I hear people wonder if it is okay to brush their teeth with tap water. It is completely fine.