How Much Does Water Cost?
I don't know if there is a RAW listing of the price of water. The thing is, there are so many skills / spells that are provided to players to find or conjure up a source of water that the game's designers simply assumed that in the Forgotten Realms setting, water is too cheap to assign a price to it. But, do not fret - we can give water a price by assigning to it some reasonable bounds that are specified in the RAW.
- A gallon of ale costs 2 silver pieces (PH, pg. 158).
- A waterskin is listed at 2 silver pieces. When full, a waterskin contains 4 US liquid pints. (PH, pg.150)
- Lifestyle expenses "cover your accommodations, food and drink, and all your other necessities" (PH, pg.150). If you live a Modest lifestyle (1 gp / day), "[y]ou don't go hungry or thirsty" (PH, pg.151). It's likely that at least 50% of the daily cost goes to accommodations alone (think renting an apartment). Conservatively, let's assume the remain 50% is split halfway between food & water. Lifestyle expenses thus indicate the daily price of water is 0.25 * 1 gp = 2.5 sp.
Again, the RAW doesn't say anything about minimum water intake for a player character (perhaps this varies too much between races). However, we can guess "not going thirsty" means you take in enough water on a daily basis to remain healthy. However, the WHO (World Health Organization) has published a study on "Water Requirements, Impinging Factors, and Recommended Intakes":
the minimum water requirement for fluid replacement for a 70-kg human in a temperate zone equates to 3L per day... for an individual the same size but in a tropical zone equates to 4.1-6L/day
-- pg. 8
Those minimum water requirements average out to 4.5L ~= 1.2 US liquid gallons. Some estimates:
- 1 gallon of Ale (presumably more expensive than water) = 2 sp.
- 1 gallon, or 2 full waterskins * 2 sp = 4 sp.
- 2.5 sp / 1.2 gallons of minimum water intake ~= 2 sp per gallon.
Purchasing from an inn? If an inn sells ale by the gallon at the price of 2 silver prices (as listed on pg. 158 of PH), then water sells for 2 sp at the most (As @JackAidley commented, NOTE: this is implementation-specific. Think about how the Inn gets its water - how difficult it is to obtain potable water will likely determine the Inn's price for water. Does it have to pay a fee for access to the city's reservoir? Is the Inn convenient built on top of an underground spring filled with potable water?)
Purchasing from a local source? If the water source charges local customers for water (say, a government-run water distribution system requires a fee for you to collect water) then Modest lifestyle expenses suggest the most you'd spend in a day is 2.5 sp, at 2 sp per gallon while Squalid lifestyle expenses suggest you might get away with less than 2 sp (in this case, I'd imagine the water isn't guaranteed to be clean for consumption).
Purchasing from a merchant? If the merchant is selling water in the form of full waterskins, the most you'd spend per gallon of water is 4 sp for 2 full waterskins at 2 sp per waterskin (as listed on pg. 150 of PH).
How Much Does Water Weigh?
Without a container, this is a simple calculation. From Wikiepdia, "Kilogram":
(Under Definition section)... The final kilogram ... had a mass equal to the mass of 1 dm^3 of water at its maximum density, approximately 4 °C.
1 dm^3 = 1 L = 1 kg (by definition). 1 gallon ~= 3.8 kg = 8.4 lbs.
Otherwise, the answer will depend on the container that is being used to carry the water. The following numbers are only useful
- A full waterskin weighs 5 lbs (PH, pg. 150). A waterskin has a maximum capacity of 4 pints of liquid (PH, pg. 153). 8 pints = 1 gallon, so 2 5-lb full waterskins carry 1 gallon of water.
- A (presumably full) flask of holy water only weighs 1 lb (PH, pg. 150).