According to Feng Shui, I suggest red. That will cover all your elemental requirements. For more precise advice on this I would need to know what rivers you boat, and style of boating: white water? flat water? which way does the river flow (that's probably the most important question)? Perhaps you get several cans and paint them different colors and use them appropriately as the trip dictates. Another good point of having extra cans is to take them all with you and keep them below decks. Use a certain color depending on the days of the week. Maybe you have some trip mates that are having friction among the ranks, you could pull out some pink ones during the trip to help increase harmony....
Or you could paint them flourescent green, that way it will be easier for me to steal them if we run a trip together...mine are that color!
Whether U Think U Can, or Think U Can't; U R Right!