My grandfather had a novel solution to the regular "borrowing" of his dinghy by drunks - he installed an inspection port in the bottom, and would take the screw in plate home with his oars.

Hydraulic hose can still be cut with a knife, or side cutters - it's just a bit more work than rope. How about a plastic sheathed stainless wire instead? You wouldn't need all of the ropes to be knife proof - just enough to deter/prevent the removal of the boat. One at each end ought to be enough? If they're really that keen on stealing it, then not much will resist bolt-cutters.