You need to determine the tongue weight. Google will provide several ways.
Motorboat trailers, due to their engines and weight at the stern, only require 5% to 7.5% of the loaded trailer's gross weight on the ball for safe towing so 7000 pounds @ 5% would only be 350 pounds. Doubtful the trailer and boat are correctly loaded if that little weight is causing quite a bit of squatting. Possible the trailer is not properly constructed for the boat. Axle placement, etc.
7000 pounds is a lot of boat. My J24 keel sailboat (with 950 pounds of lead in the keel!) only weighed 3100 pounds and the trailer maybe another 1000-1500.
What brakes are on the boat trailer? Surge brakes are incompatible with many WDH systems as is the "pole" (vs A-frame) tongue.
-- Chuck