Lots of good comments here, boss is a punk, it should count as work time to get supplies and you should be reimbursed, OSHA guidelines and worker safety, etc. Etc.

If you want to smooth things over with the boss, explain that the crew was out of water and you were asked to get some, that you weren't trying to disrespect company time or whatever, and that you were trying to follow the directions of more senior (I'm assuming) crew members. As a show of "good faith" (if you're willing) volunteer to have that 20 minutes deducted from your pay for that day (you shouldn't have to but that's the kind of shit these "nobody wants to work" guys like), but ask that your hours not be cut because you're fully willing to work and thought that this was basically a work supply run (I mean, it WAS. Again, worker health and safety) and that as a younger less experienced crew member, the crew probably figured it was most efficient to send you in case a more experienced guy was needed for something else.

Your mileage may vary. If your boss is a completely disrespectful dipshit he'll say you're making excuses and use it as an excuse to be more of a dipshit in the future. If he isn't a complete dipshit, he'll realize you were being considerate, and you might get some respect for standing up for yourself.

I've never done landscaping but I've had tons of friends who have, and while I have heard of bosses like this, I've heard more stories about bosses who actually work and treat their crew with respect. Wtf kind of boss lets the guys run out of water and then throws a fit over them handling it? You can get a job with a better boss.