Outboard motor for dingy
The environmentally safe way to go is with a 4 stroke. But beware! They weigh A TON! I have a Zodiak 310 (10''6" or so) and the local dealer talked me into an 8hp Honda. This is really a tremendous OB, but at almost 90 lbs... I have had to rig a hoist to move it from stern rail to dingy and back. Quite a change from my dear old British Seagull. Or course the Seagull was not exactly the environments best friend.
There seem to be some really nice Mercury 2 strokes in your power range, light weight and effecient. But they are no longer available in all states, as emmissions laws tighten down.