Sounds like a moose -- especially with the tracks leading to the water. They are good swimmers.
As for the tracks, here's a decent shot of a moose track in dried mud, with my foot (size 11) as a reference point:
http://www.pbase.com/gfevans/image/38566396
And as for the, er ah, *droppings* mentioned above, they look like this:
http://www.pbase.com/gfevans/image/38566398
(And don't ask why I pay for online storage space for digital photos of moose dookie...)