Assuming you're talking about the exterior of an aluminum canoe, the best thing to use is a mixture of dish soap and water. You can also use boat soap, but it's not necessary. Just make sure you rinse the canoe thoroughly afterwards.
If there are any stubborn stains, you can scrub them with a soft brush or sponge. For really tough stains, you can try using a mild abrasive cleaner like CLR or Bar Keepers Friend. Again, just make sure you rinse well afterwards.
Once your canoe is clean and dry, it's a good idea to apply a coat of wax to help protect the aluminum from corrosion and UV damage.