I'm in the same boat in trying to turn over water costs to the tenant. I recently listened to podcast #60 w/ Serge who turned a 32 unit I believe and sub-metered everything. His method required zero money for installation, and the fees from the company were charged to the tenant so it was pretty much a no brainer. The company basically setup the meters, and acted as the water company. Owner was still on the hook for the water with the city, but reimbursed through the company. Upon moving in, the tenant had to call in to 'turn the water on', and if the tenant didn't pay, the water was shut off.
I'd suggest having a plumber look at it, and figure out ways to attach a sub-meter with a lock box type of installation. Sometimes it's just a plug and play, or it may have to be a big plumbing job. Just depends on the configuration of the plumbing and how it's set up.
I would just call the water company and ask them any questions you have and call a few plumbers as well. That should lead you down the right path.
You could start with energy efficient faucets and shower heads. Some energy companies offer them for free or discounted rates.
First place to start is putting a lock on the outside water faucets. Then stop any plumbing leaks. Then replace shower heads, faucets and toilets. Then look into submetering or RUBS.
You can't just submeter/RUBS and bill back to tenants if that is not in their lease. You can do that and then wait to bill back to tenants until their leases come up for renewal, but then you risk them not agreeing and moving out and now you have a vacancy.
I would replace any old 1.6 gallon per flush toilets with newer 1.2s. You could retrofit the old toilets instead of replacing them with a kit that costs about $30/toilet.
You may also want to check all of the toilets for internal leaking/running and replace the flappers of any toilets that are doing that. Also check all of the sink and shower/tub faucets for drip leaks. Tenants won't inform you of these things unless they are an inconvenience (they aren't if the tenants aren't paying the bill).
Low flow shower heads and faucets are also a good idea and will pay for themselves quickly.
I would also close the valves leading to any exterior hose bibs and change out your landscaping to plants that don't need extra water. You can set up rain barrels to catch water from your downspouts to help with gardening.
Replacing 3gal toilet with 1.2 gal toilet is what is in my mind.
I am not sure on what submetering is.
How much it cost for submetering ?.
Is it economical to install separate meters for all units