Hi Matti, I agree that Dead Weight is one of the weakest earlier episodes. The payoff clue isn't great either.
The term "dead weight", refers to something that you have to lug/carry around with you that has no value. For example, let's say that your favorite soccer/football team has an aging player with diminished skills who must be paid for three more years at a high salary. This man doesn't help younger players, he is bad for team morale and fans don't like him so he won't help the team sell more tickets. The team is stuck with him taking up a roster spot and a lot of money. That's dead weight.
I think the title of the show comes from the murder victim being weighted down to the bottom of the ocean. I hope that explained it.