The gas in my boat is 18 months old,it has stable in it is it usable? Will PRI fuel additive help? or should I get rid of it?

I'd say depending what motors and compression your running, and the octane you 'need' to run, but I heard it drops an octane point or 2 every couple months while it sits.

Personally, I'd dump it start over. Last year dumped @175-200gals Didn't want to chance my new blower motors with 'old' fuel and that was only 5mo. old. Cost me $500 and sucked, but better than melting something and having engine builder tell me "I told you to dump it"

Sell it to a landscaper they can burn it in their lawn mowers.

The answer to your question is NO

run it in your car / truck - for gods sake don't dump it.

Thats "liquid gold" these days! As long as you added another batch of stabilizer in it after 12 months, it should be ok. But if you are going to use it, it would be a good idea to add more stabilizer to it first. I believe stabilizer ultimately includes an octane bost. Or, as stated above, sell it to a landscaper, etc.
I have had Diesel sit for that long with stabilizer in it and it worked just fine. Just my .02

Retard the timing a little, then add some Toulene to it.

There is no way I would run hard with it that old. Run it REAL easy tell you can get new gas in it then run it harder and harder pregressivly.

Add some premium and move on

Check its flammability with a lighter if it goes, its good!