No matter which way you cut the concept. No one wants an extra bill.
so I would go with the pain of the Water/sewer sub bill.
WHY?
long term it's a business benefit. You can, over time, manage water waste, and with first year active management towards leaks. You can reduce it to nothing, giving the tenant peace of mind.
What you do is the global install, look for water usage that is outside the statistical norm.
that's your first hard leaks. Go into the units, find the problem, document what the fix is most likely. then book the plumber after all ( outside of norm) units have been investigated.
Boom, 50% or more of your leaks and water waste have been found and resolved.
Now, click it on to billing, watch the calls come in over the next 90 days to get water repairs done.