Dehydrated water. Or compressed water!
Both explanations being ludicrous - water does not compress easily at all. But if you have a magically strong reservoir, and a magically powerful way to pump more water in, then maybe... Could explain the force behind the water blasts too.
Actually I think you'd need a two-reservoir system. A tiny high-pressure reservoir connected to the hoses, fed by a large absurdly-high-pressure reservoir for storage.
For a rough idea of the pressures we're talking about: Modern nuclear submarines seem to have a lowest depth of about half a mile before they're expected to crumple. If we go twelve times as deep, six miles under the ocean, the water is only about 5% compressed. I haven't watched the show in a long time, but it sounds like we need *at least* 100% compression. Probably more like 1000%.
So we're talking about an adamantine reservoir with adamantine mechanisms for refilling and delivering the water. Refilling would take astronomical amounts of energy, almost all of which would be wasted on delivery... The potential energy is there in the tank's pressure, but the water cannons clearly don't use it. Perhaps an adamantine turbine system reclaims the energy and stores it in a magically high-capacity battery to power the refueling process.
Or uh, yeah maybe there's a portal to the elemental plane of water.
You can't explain Pokey mans.
You should have said so earlier, ninja!