Let me tackle these one at a time:
is a leaded fuel, and is more precisely refined than regular unleaded. It has chemicals that make it last longer and it won't break down or attract water like regular unleaded. And for good reasons; when your outboard stops running, tear into the gear locker and look for the paddles.
When your Cessna stops, quickly
look for a straight section of empty highway.
AVGAS is higher octane, but for engines that normally take regular unleaded, it won't help performance. Running AVGAS in any engine that is marked "unleaded only" violates EPA and California Air Resource Board regulations. AVGAS can destroy a catalytic converter, foul spark plugs, plug spark arrestor screens, though off the oxygen sensor, and leave lead deposits on valve seats. Not all those are an issue with most outboard engines.
can absolutely help; they act as toxins to microscopic bugs present in all fuels. These bugs reproduce by the millions in tanks with a lot of air (oxygen) inside that are in a warm environment. Enough dead bugs and bug poop react with the fuel and you get that nasty varnish odor along with "biomass" that can plug up the carburetor jets, emulsion tube and tiny passageways. The general rule of thumb is, use a fuel stabilizer/treatment if storing fuel for less than 90 days. Otherwise, dispose of it by dumping into your vehicle's tank.
• Most marinas sell unleaded fuel that is 100%, e.g., no ethanol. A wise choice; you get the best of everything and still stay legal.
• Going with a slightly larger jet in the carburetor may help; larger jets have bigger holes, make the engine run a bit richer, and are less likely to clog. [Caveat: using a larger jet probably violates a few emissions regulations.] Honda part numbers contain a secret code to tell you the jet size. The standard jet for a BF5 is a #75, and the part number is 99101-ZH7-075
0. The next size up is a #78 jet, so get part number 99101-ZH7-078
• Not going to use the outboard for at least the next 2 weeks? Drain the carburetor bowl.
IN THE FUTURE
, all outboards will have fuel-injection and will eliminate the problems of clogged-up carburetors. Honda has a 125cc scooter engine right now that is fuel-injected, so the tech is there, but the cost is still an issue. But, we're getting closer every day...
Caveat: I work for Honda, but the preceding is my opinion alone.