For '06-'10, the 53G HD Towing Package provided:
1. Upgrade from 4-flat wire connector, to the 4/7-pin combo connector. You may not need this for your boat trailer.
2. Upgrade from welded Class II 1¼" receiver, to welded Class III/IV 2" receiver. You are stuck at 3500 lbs max without this upgrade (or the bolt-on underneath).
3. Transmission Oil-to-Air Cooler: On Ford Towing Guide, the Standard column has "X" indicating it has a cooler. 53G column shows "Std". Not sure why the different indicators. It could mean the 53G uses the same "Std" cooler as the base vehicle. Or it could indicate it's actually something different. I would physically compare your '06 to the '08 to see if there is a larger inline cooler installed on the '06.
4. V6-only: Change from 3.55 to 3.73 axle ratios to protect the 5-speed drivetrain at low-speed, high-load. Not needed (nor available) for the V8/6-speed, with it's super-low 1st gear.
My gut says that the welded receiver in the Class II vehicle is every bit as strong as the Class III/IV, since it's an integral part of the truck's frame, and partly responsible for the increase in frame rigidity for the 4th gen vehicles. Again, get a look under each truck, and see if you can see any differences (other than the size of the square opening. I say this hesitantly, but for a boat trailer that has it's own surge brakes (I assume), and a TW less than 500 lbs (no WDH needed), I would be tempted to tow your 4200 lbs with no changes, as long as you can confirm the trans cooling question.