Towing is a lot more than just the tow rating of the vehicle you have vs. the trailer weight. First off, boats are weighed dry, with no gas or gear. They hold a LOT of gas, and you always put gear, coolers with ice, etc....you can easily add 500lbs to it without even trying.
Then, you have to add the weight of the trailer. That's easily going to be 1000lbs for a 20ft boat, if not more.
Now, you also have to consider the tow vehicle: Most tow ratings are simply for advertising....if you read all the disclaimers and footnotes, you'll find that pretty much no vehicle is capable of towing the advertised "Max Trailer Weight" it's rated for.
Tow rating is based on the base model vehicle, with no passengers, gear, cargo, or usually even gas. And a 150lb driver. Who weighs 150lbs?
And who buys the very base model of anything? So everything you add to that vehicle....options like power windows, skid plates, sunroof, etc....they all take away from that vehicle's Gross Vehicle Weight Rating, (GVWR) and from the Max Trailer rating.
Now, you add the weight of your passengers, and any cargo or mods you've made to the vehicle.
Not to mention the tongue weight of the trailer....that counts towards the Payload capacity of the tow vehicle....another rating you have to be careful not to exceed. A 3000lb trailer will easily have 300lbs or more of tongue weight.
So, what to consider when you are figuring what your truck can tow:
GVWR....max the tow vehicle is allowed to weigh. Go weigh your truck as it sits, with a full tank and loaded, and subtract that from the GVWR. Then you know how much cargo/passenger/tongue weight you can add to it.
GCVWR...Gross Combined Vehicle Weight Rating....max your truck and trailer are rated to weigh, fully loaded. Gotta weigh 'em both to get this one.
Max Trailer Capacity....max anything you tow is ever supposed to weigh.
You aren't supposed to exceed any of these numbers. In fact, most experts will tell you it's much better not to exceed 85% of the max. rated weights.
But for the OP, it's pretty obvious that you can't tow that boat without a bigger truck. You don't need to do the math I've laid out here.