7 units of water would mean that they would have to have the same volume as a square in game. They should hold the same amount as booze barrels do.
Which does, in some way, make it seem odd. A barrel of alcohol can have upwards to 25 units of alcohol (one unit per drink); water in a tile is 7-drinks deep (we know this because a flask or a skin holds up to 3 drinks of water or booze, and I believe is subtracted equally from the water source). We'd either have to redefine alcohol units or water units, or leave the two be; if we go for the lattermost, then a barrel should store the same amount of water as it stores booze with no quality on it (i.e., 5 units).
Of course, water has few intrinsic properties that makes it a commodity, especially a traded one (the assumed technology implies that stored water will go stagnant, requiring it to be cut with booze to keep it "fresh", i.e., grog--but doing this all the time to stretch out your booze reserves some seems like a pointlessly frustrating endeavor). The only in-game aspect I can think of is for centralizing water for farming, especially early on if you don't have an available water source and don't farm above ground (it's much easier to farm above ground, because most ground plants can be easily collected at first, processed into booze, and the repeat with seeds on plots, and then you have a stable above-ground farming system. Unless, of course, you embark into a place with no surface plants whatsoever).
In the end, though, you replace pit/pond dumping with stockpiling. But is that really necessary for this game?