My opinion, take it for what it's worth (not much):
1. If someone has a gun on board it should be in a locked cabinet when not on the person.
Legally? That depends on the state. I also don't know what will happen if you get boarded by the CG. Personally, I'd prefer they keep it on their person, I don't make a habit of screwing with other people's guns. If they are irresponsible with it on your boat, it may come back to bite you and not them (or both of you). Keeping it locked is your safest bet, but depending on where you are, it may not be legal (IANAL), BUT...
There is something called the "Firearm Owners Protection Act" that allows safe harbor for people traveling through states with weapons. There are requirements on storage and limitations on how it applies. I suggest you do your own research on it to see if it applies and how to abide by it.
As far as safety goes, that's probably the best bet. I wouldn't worry about unloading it, depending on the type of gun, that's a matter of preference. Unlike what's displayed in movies, guns don't magically go off when dropped or hit. Are you safer to unload it? Probably, but I wouldn't consider it a big concern.
2. There is some legitimate room for disagreement about whether the gun should be loaded and if the ammo should be stored with the gun.
FOPA has requirements to be met if you aren't going to be arrested for having a firearm in another state. Read to see if it applies here and how to abide by it.
For safety, I wouldn't worry about them being together or even loaded.
3. When handling the gun the finger should not be near the trigger.
This is a basic safety rule, I usually keep it alongside the slide on an auto and down the front of the trigger guard in a revolver.
4. When handling the gun the barrel should not be pointed at anyone even when unloaded (duh).
"Don't be a dumb***"
5. I should not be handling someone else's gun without permission except in extreme conditions. (I knew that)
Make sure you establish ground rules for this before they board, so everyone is on the same page.
6. If the gun owner wants to show me is gun he should unload it before handing it to me.
Yes, they should make sure it's safe before handing it over to you.
6. If the gun owner appears unstable or cavalier about the gun get off the boat. (I knew that too)
If they are unstable about a gun, I'd consider them unstable and not want them aboard anyway, gun or not.
As for item 4 above the way I understand it the barrel is pointed towards the floor most of the time. In a boat however that may not be optimal.
Not necessarily the floor, just a safe direction. Keep in mind though that bullets can travel far and through many surfaces (including drywall and fiberglass), although with handguns their efficiency diminishes with distance and rather quickly.
Go through extreme pains to make sure the gun is unloaded and safe before you handle it. The general rule is to keep it pointed in a safe direction (e.g. not at people). This begs the question, what are you doing with the gun that would require you to handle it loaded? If you are unloading it, keep your finger off the trigger, drop the magazine and eject the round in the slide, leave the slide open, pick up the round, and store it back in the magazine. If you are doing anything else (e.g. dry firing), you should probably do it somewhere else if there is nowhere safe to point it and do it at. I also can't imagine anything else you'd want to do with a gun on a boat, aside from unload and store it or retrieve it. I wouldn't worry about pointing it at the floor or side of the hull while I go through this maneuver, just be aware of where you are pointing it and what is behind what you are pointing it at.
I would think that on a passage of several days it should to be cleaned and oiled every few days. So while you are handling it how do you orient it while cleaning for example?
Depends on the gun. Some are made for more extreme conditions and will not rust easily, just make sure you keep all the moving parts oiled and swab oil down the barrel (this is the most common place for rust and pitting due to corrosion). There are usually common wear spots on guns and you can tell where they are after about 500 rds through the gun, you'll see the finish starting to wear off in certain areas, be sure to oil those really well. I'd also rub down the entire gun with a silicon rag.
I'd be more cautious about taking older steel framed guns, though. I've seen some revolvers rust really quickly even on land.