I'd offer them cold water to drink, and otherwise, not worry about it. I didn't tip the guys who installed our carpet either, but they did an excellent job, so I did call the company and make sure they knew that these guys were a great team who did great work.
I don't know about the tips. We offered them drinks, food and even offered for them to have lunch with us. Ours was a really quick job and they were done before lunch and had made other plans because they knew they would be done before noon. They did take us on the drinks though. They seemed like nice guys.
I'd offer them cold water to drink, and otherwise, not worry about it. I didn't tip the guys who installed our carpet either, but they did an excellent job, so I did call the company and make sure they knew that these guys were a great team who did great work.