I'm just upgrading the hitch on my Sunseeker 3010 DS to 7500 lbs. it is rated to tow that by Forest River and the upgrade is a FR part.
How far will you be towing the boat? If you are going through mountains, I'd probably be mildly concerned. If it's a flat land tow, you'll probably be all right as long as you boat trailer brakes are setup properly.
So if I were to tow yours boats on mine, I would weigh 20500 lb versus a max of 22000 lbs.
There are other Sunseeker models that weigh less but have the same towing capability.
So you might get one that weighs 12000 lbs loaded and only be 19000 lbs going down the road, same as me towing 5500. Lbs.
19000 lbs gcvwr is no problem for the e450 ford.