I have one of the 28hp copperheads (50hrs) on a 1542 lowe flat bottom and have been fighting with it vibrating/buzzing the whole time ive had it. Its broke 2 new mufflers (one under 25 hrs and the other under 5hrs) and its broke the muffler support bracket on the back of the block as well.
My dealer has sent me 3 different props and put a whole new lower unit on it but still no improvement.
I know a loose belt and bad bearing can cause vibration but he claimed he checked it when he put the new lower unit on and it had a brand new rear bearing with the new unit, and yes i keep it greased
I know mud motors vibrate some but i dont think your hand is supposed to be numb after 10-15 minutes of riding.
Any help is greatly appreciated

LONG SHOT---check for loose fly wheel, dirt dobber nest on, under fly wheel, on cooling fan fins, and also look for missing or loose magnetics under fly wheel. Any of those will cause an out of balance and vibration of the motor.
Last vibration cause I found was the damn big clump of mud them damn dirt dobbers had built on my fly wheel. Made me think it wasn't firing on both cylinders.

Im out here looking at it row and everything aeems to be tight, so i dont have a clue where to look next. I appreciate it

If it has the grass catcher on the flywheel, take all that out.

Does it vibrate bad on trailer or just in water?

Doesn't really vibrate on the trailer. I checked the belt myself and it was a little loose, i shimmed it up and it helped out a little