According to the manual, the 2000 lb rating is a 2000 lb "single line pull". That means that if the winch were mounted in a high place and a 2000 lb weight were directly below it, it would be able to lift the weight. (Vertical lifting is prohibited for this winch.)
Note that the 2000 lb rating applies only when you have the minimum allowed 5 wraps of cable on the drum in a single layer. As soon as you have pulled 6.5 ft of cable (or if the cable overlays an existing wrap, whichever occurs first), the pulling force drops to 1630 lbs because of the reduced mechanical advantage of the now-larger drum + cable layers.
When the drum is nearly full, with 6 layers of cable on the drum, the pulling force is reduced to 940 lbs. See page 2 of the manual.
If you will never need 50 feet of cable for the expected use case, you will get greater pull by cutting the cable shorter, leaving enough cable to wrap 5 times around the drum with the cable extended to the longest pull that you will ever need.
As an alternative, you can nearly double the pulling force by using a snatch block attached to the boat with the cable's hook connected to your truck's tow hook or some appropriate hard point near the front of the trailer. With 50 feet of cable, that may be your best option. See page 9.
As far as rules of thumb for real-world capacity in a real world task of pulling a boat out of the water: you will develop one. Everything you learned in high school physics regarding pulleys, mechanical advantage and inclined planes goes right out the window when you introduce friction, and that is entirely dependent on your specific conditions such as the slope of the boat ramp, the sliding friction between the boat and trailer, the weight of the boat and the adequacy of your power supply and cables.
