The short answer: Tin snips (or a wire cutter) for the wire, and a sharp blade for the bulk of the hose material.

Marine hose used beneath the water line should always be wire-reinforced. The first time you attempt to cut it, your first inclination might be to grab a sharp knife and go at it. And then you hit the wire, and realize you're doing your blade a terrible disservice. At this point, you might consider using a hack saw. While this works, the more "surgical" approach is to cut straight across the hose until you reach the spiral/diagonal line of the wire, snap through this with a stout pair of wire cutters (bending the wire back and forth a couple of times can help), and then complete the cut with the blade.