We feel you should be proactive and offer a refund before they ask for one. If you apologize sincerely and then say something like "We feel it is only right to compensate you for the inconvenience and we will refund X day's rent."
They did call your own maintenance people, even if they didn't call you so they did the right thing. However, how bad was the water pressure? A mere trickle or just less than normal? Not that you should bring this up or quibble about the severity of the issue. Since it bothered them, just apologize strongly and give them the refund.
Assuming they didn't pay the plumber, definitely offer maybe 2 day's rent of the 5 days they had this problem. After all, it couldn't have completely ruined their stay. It may depend on how upset they seem and from what you say, they seem to have coped okay so should be happy with an offered refund.
Thanks for your input - I took your advice on being proactive, offered to refund 2 nights for their inconvenience, and they were very happy with that, and actually want to book again next year.
As a side note - if you own rental property in an old complex, you could run into these same issues - prior to our pulling out of the condo's rental pool last fall due to severe mismanagement, the condo manager oversaw the installation of the new tub and shower, as there was a leak somewhere in the wall, and they had to replace the tub after tearing it out. The plumber we have contacted worked for the maintenance department at the condo complex, and the Manager told him that he was not to replace any of the old equipment, including the valve, which is now our issue. If they would have contacted us, we would have paid for it!
I have a same day turnaround, and have to rent another unit for my new guests while the shower is fixed. I guess we should have asked more questions when they were doing the tub work, but I never thought they would only do it halfway, so to speak -