there is big pike in the lake, and close to the river is a great spot to start hunting them. There are lots of smallmouth bass in that lake as well. its not uncommon to get into half dozen smallies in half dozen casts. To start i would fish the islands, the windswept side.
If you fish the docks out by beumaris you can get lots of good fish as well. Not many on millionares row fish, so it its pretty productive. spinner baits were the best bait the last time i fished muskoka, which was a few years back.
If you are around gravenhurst at lunch time head to boston pizza for lunch, its right in the water and you can dock your boat there. Don't even try on the weekend though, tat lake can be a zoo at times.
CHeers!