The Lifebelts
Lifebelts were made to insure the survival of passengers who thus
didn't sink into the water. Rose's lifebelt doesn't help her and is rather making her moves difficult
(see how she swims to the wooden board). Jack doesn't wear a lifebelt, and it makes him sink,
just like the ship, when Rose lets go off his hand. I consider it as a symbol: Rose admits that Jack
is dead but that she all the same can keep going and living.