This article is from the Boats FAQ, by John F. Hughes with numerous contributions by others.
Assumining the outboard is a two-stroke, Yes. In fact, it is
prefered. Lead is in fuel primarily to lubricate the exhaust valve and
valve seat in a 4 stroke engine. The two-stroke has no such valve or
seat and so requires no such lubrication. The lead compound also
served to prevent pre-ignition, or "knocking" or "pinging". This has
long since been resolved in unleaded fuel and so is not an issue.
Lead in fuel causes fouling of the spark plugs. No lead, no lead fouling.
(Though oil fouling may still be a problem.)
Leaded fuel is only available in "regular" (at least here in the Northwest
USA). Higher compression outboards that require higher octane fuel often
have problems with the leaded fuel now available. Unleaded comes in
"super", or high octane ratings. This is the recommended fuel.
The above information was obtained from a phone-interview with a
long-time outboard mechanic at Chic's Outboard Service; 2043 SE 50th;
Portland, OR; (503)236-8970, and has been paraphrased by R.C.