Jack and Rose spend a fair amount of time below decks, as the rising water floods the ship. That water is ocean water, so it shouid be just a few degrees above freezing, at best. But there is no indication at all that they're in brutally cold water.
Additionally, given the lightweight clothes they were wearing, they should have been shivering cold and looking like they just spend 15 minutes immersed in freezing cold water.
This is the most glaring goof that I see when I watch the film-it is so obvious.