What kind of boat (i.e. size, power, fuel capacity)?
The only real choices are Munising on the west and Grand Marais on the east - they're about 40 miles apart by water. No real options in between if the weather goes sour.
The real interesting rocks are closest to Munising, which is what I'd recommend - if your boat is up to it. If the weather is dodgy, there are some beaches and sand bars in the bay at Munising that can be fun.
This is the Pub. Leave common sense at the door.