What species is the leaner? Any large branch scars or any other source of heart rot? How far down is the water?
How often do you get ice or sticky snow?
My assumption, and I knew I shouldn't have done it, was that the 'floor' would decent gaps to at least let out rainwater, similar to any other deck (and to save a few pounds).
I did NOT think about OP living in Michigan...
OP, build the floor at a 45 degree angle to the earth to allow the snow to slide off. Your kids will love it.