First of all you are picking up pennies while the world is burning bills. The amount of oil from your chain is small even compared to the amount of oil that comes out of your exhaust, which in turn is small compared to the unburned hydrocarbons from the gasoline, which in turn is small compared to what everyone in their SUV's are releasing. You are already a good citizen.
But that's not what you asked.
Take a look at some of the products at bicycle shops. One product I like is a graphite powder in mineral spirit solvent & oil. The spirits evaporate, leaving the graphite with enough of a film of oil to keep the graphite from running away. There is similar product that uses Teflon. A friend who is an ardent cyclist claims that for these to work well, you have to remove ALL the oil from the chain. This usually means two washes with solvent, then a wash in acetone to remove the oil film left by the solvent. But talk to the guys in the bicycle shop.
Caveat: The forces between chain and tooth are a LOT smaller on a bicycle than on a motor bike. If this doesn't work you may need two new sprockets and a chain.
My estimate is that the graphite would work better. The key for it is keeping it attached to the chain.
For an outboard motor, the key is keeping the seals intact. Also: Many outboard motors discharge their exhaust under water. This is NOT friendly. Run your motor in a barrel of water for 20 minutes, let the water set overnight, and look at the reflection in the morning.
Almost all pollutants are tolerable sufficiently diluted. Using a tiny put-put going through a swamp that sees a dozen boats a year is unlikely to be an issue. Take close look at the shore line, however at any marina.