I saw a video of Bill Dance and a fishing buddy do that with a smaller boat. Was funny as heck. Bill was driving the boat around with the trailer still on and drove it back up on the ramp to get re-connected to the truck.
I don't think the weight of the trailer is enough to sink most boats. I don't have that much experience but figure if a boat is rated for 7 folks at 200lbs each (ok..so they aren't real people) that's 1400lbs plus a safety factor that the boat is RATED for to operate normally/safely. Figure in the safety factor and I'd guess you could probably go 3000lbs or more in an emergency....that's a total guess tho and would require calm waters..etc.
Somewhere there's a calculation for water displacement vs weight capacity. Doing the calculation tho requires you factor in the weight of the boat/motor and all equipment on board including wire/fasteners..etc.
"Our greatest happiness does not depend on the condition of life in which chance has placed us, but is always the result of a good conscience, good health, occupation, and freedom in all just pursuits." - Thomas Jefferson