Don't listen to H2gayperson.
First, some things need to be established. What size boat, type of trailer tongue, does it have surge brakes, etc.
If you have the rear air you don't need to expect the rear to sag any. If not, you need to think about how much tongue weight the trailer has.
You can't simply just go to walmart and get a hitch. Many of the hitches are designed for 5K pounds or less. You would be better served to go to an RV store and talk to them about it.