A wrench is accidentally dropped from the top of a mast on a sailboat. Will the wrench hit at the same place on the deck whether the sailboat is at rest of moving with a constant velocity

Yes, because the wrench is moving at the same speed as the sailboat.

The main difference is that a person on the ground would see the wrench moving diagonally, while a person on the boat would see the wrench falling straight down,

This difference in paths lead to the relativistic change in lengths.