The Spirit Anglers realized that, although water can be conjured from thin air through magic, some adventurers are more connected to the physical world. Water isn't stored in this pump, nor is it routed through the pump. The pump simply calls upon the same source of magic that you also do when casting a water strike, or humidifying buckets of water. Not all of us are so in touch with magic, and so we have a pump. The runes that power the pump are re-stocked by the Spirit Anglers between rounds at Tempoross Cove.
That'll be $11.