my experience up there the past few years is that there's more risk at slow/idle speeds than running wide open.

when driving high speed i tend to stick to well-know runs or the marked travel routes. it's when i get off those and start exploring around that i've found some rocks. in both cases i actually hit the hull of the boat rather than the motor/prop, but the lesson i learned is to put somebody up on the bow of the boat to be on the lookout!