If you're tired of hearing "What's that smell?" from the passengers on your boat, it might be time to check out Odorlos, a holding tank treatment that eliminates all odors while breaking down waste. Four ounces will handle up to 40 gallons of waste, using natural and environmentally friendly products. Odorlos also lubricates the valves and won't harm the tanks on your boat. A 40-ounce bottle costs $11.99.