I think that for moving water or boiling you have a heat transfer coeficcient of 25kW/m^2/K and hence
this thing should be able to produce 1-10MW of heat.
There's no definitive answer, as it depends on pressurisation and how fast the water is flowing, but you can push 1MW/m2 into still water before the Leidenfrost effect causes problems.
(This limits the deltaK in the heat transfer equation to ~80C at 1atm)