Even without magic, the uru metal that Mjolnir is forged from is hardly weightless. It is one of the densest, strongest metals in the Marvel Universe. So, that must be taken into account with talk of sails and floating.
Robots and other nonliving automatons can move Mjolnir. Following are three comic images that I had easily on tap. I can think of other moments, but don't have the citation evidence readily on hand.
Journey into Mystery #86 shows a robot making grab.
The Mighty Thor #210 shows magnetic (part mystical?) force "lifting" the hammer.
The Mighty Thor #355. Discusses nonliving creatures doing the heavy lifting.