Northguy,
Canoes fly just fine on the floats. Secure them to the struts, and take one canoe at a time. Pay attention to how long they are, and ensure that they don't conflict with prop and/or water rudders/other rigging. I used ratcheting cargo straps to ensure that I got things nice and tight. You really don't want a canoe coming loose in flight......
As to legalities, the Alaska region of the FAA permits carriage of external loads, under a Restricted category airworthiness certificate and a permit from the FAA. They issue these pretty routinely there. There are one or two other districts of the FAA that permit them as well, but for the most part, not legal in the Lower 48.
If you're concerned about that, Pak Boat makes some really nice folding canoes. I used to carry two of them, folded up, one in each float compartment of the Baumann floats on our Husky. Much simpler than strapping stuff on the outside in any case.
As to lumber, ie: Plywood, I have flown lots of wood on the floats. There are some things you need to pay attention to:
First, again, use ratcheting cargo straps to secure the stuff to the struts. I would NOT carry plywood on the spreader bars in full sheets. The problem is that in that location, the plywood affects the airflow over the horizontal tail.....which may or may not be good. I've done it, and won't do so again.
Full sheets of plywood won't fit on the struts of a Husky, so I cut them lengthwise in half. Now, they are 2 x 8 ft. and fit fine.
Be VERY careful of weight and balance. Plywood is heavy and dense. And, full length, it can extend aft quite a ways......balance.
I used sheet rock screws through the sheets to tie them all together. You REALLY don't want a sheet that's in the middle of a stack to start sliding aft...... I know guys who used C clamps to squeeze all the sheets together, but I want that load to be ONE piece.
So, again, be advised that, depending on where you're planning this little operation, the FAA may frown on such antics. Aerodynamically, however, the airplane doesn't really care much. Start off slow, and light, and work your way up.
Lumber gets heavy quick, though.....
MTV