How much water does a passenger airliner carry?
It is going to vary according to flight mission, passenger count, season, or what part of the world the flight takes place. 

Here are the capacities from a company document.

MD88/90 47USG (177.9L)
MD11 250USG (946.4L) Estimated

732/733 40USG (151.2L)
737 60USG (226.8L)
747 3x 110USG = 330USG
757 66USG (249.8L)-Fill valve closes automatically at 50USG (189.3L)
762/763 120USG (453.6L)-Fill valve closes automatically at 102USG (385.5L)
763ER 160USG (600.6L)
764ER 224USG (847.9L)
772ER 300USG (1135.6L)

CRJ 200/700 31.5USG (galley 8USG/30L, lavatory 5USG/19L, toilet 18.5USG/70L)


As mentioned, the term "potable" water seems to be a minor overstatement. Some companies warn employees to not uplift any potable water overseas (which means not outside the US, Canada, or UK). But based on what I've heard, I wouldn't drink it wherever it came from. 

Why You Should Think Twice Before Drinking Tap Water

Why you should never drink water on a plane, according to a flight attendant

Airline's drinking water fails hygiene tests

The "potable" water is usually chlorinated water, but you´ll never know about the truck or tank service overseas. Usually the galleys contain an anti-biological filter (silver ions) for the taps and coffee makers, which get changed regularly. Also the tanks will be disinfected every few weeks, normally during an A-check. Part of a C-check is to remove and test them.

On a B737, the potable water tank is in the aft cargo pit behind the last wall under the aft galley just right in front of the pressurized bulkhead. The tank itself is made out of composite materials and fiberglass. The tank is about 4 feet long and 2 feet tall. The 737s only have one potable water tank. 

The B747 has three 110 gallon filament-wound fiberglass tanks attached to the forward side of the center section front spar (rear bulkhead of the forward baggage compartment). The water is moved from the tanks by pressurized air. This pressure is provided by electrically driven air compressors mounted next to the tanks. Not all operators use all three tanks and some have deleted some tanks. 747 AMM, 38-11-00, page 1, pg 2. 

Passenger MD-11 hold up to four 63USG tanks, the freighter usually has all removed except for one. This water also serves for flushing the vacuum toilets if installed. The tanks sit in the utility tunnels (behind the sidewall) of the forward cargo compartment.