I've paddled it before shortly after ice out. No current to speak of, so no worries there. Grassy bay can be quite the wind tunnel at any time of year when the wind is up, however.

Early may last spring the water was too high to permit upriver travel on the Petawawa out of Grassy Bay, so I re-routed via Mckintosh to get to my car parked at Magnetawan Access.  No problems coming up the creek even during very high water. And above Misty, pretty tame flow even in high water.