free flow of water; unlimited water supply
Often used for hot springs, baths, or water services where water flows continuously without extra charge or restriction.
This inn has an unlimited supply of hot spring water.
A shower with free-flowing water feels great.
飲み放題 is all-you-can-drink, usually for beverages; 出し放題 is about continuous supply of water, not consumption.
Compound of 出し (continuative form of 出す 'to let out') + 放題 (suffix meaning 'as much as one likes').