The slang term "dead fish", when referring to sex, means when one partner just lies still doing very little during sex. It can refer to either a man or a woman. It doesn't have to refer to complete stillness, but rather is often an exaggeration of when one partner is very lethargic or unenthusiastic during sex.
"Dead fish" also can refer to kissing, specifically when one person inserts their tongue into another's mouth and leaves it there very still and lifeless. So in this case, "dead fish" refers to a bad kisser or bad kissing technique.
"Dead fish" also has unsexy meanings, the first of which simply refers to a very limp handshake. Another unsexy meaning of "dead fish" is when it's used to refer to an individual having no energy or enthusiasm, as in when someone refers to themselves as a "dead fish" after a long, tiring day. "Dead fish" can more rarely refer to when all of a person's limbs go limp and they fall to the ground.
Of interest, back in the day mattresses were stuffed with hay, or were sacks filled with hay. This is thought to be the basis for expressions like "hit the hay" or "hit the sack."
Speaking of dead fish, on April 8, 2013, it was reported that about 550 pounds of dead fish were found rotting in a river in Shanghai's Songjiang district over the first week of April. While no cause is known for certain at this time, the fish have been removed from the river and authorities have stated that tap water in the area is still good to drink.