How can I monitor water usage for just my portion of a shared property?
You will want to be very careful about installing and changing things on the property since you are not the property owner. If it comes down to needing to have individual water meters at each unit the best thing would be to work out an agreement with the owner to switch over to actual usage billing.

Part of the negotiation process could be agreement that the various tenants will chip for part of the cost of the meters.