Complex sentence: After the ship sank, Rose and Jack clung onto a piece of driftwood for almost a half an hour; there was only enough room on the wood for Rose, so Jack kept most of his body immersed in the freezing cold water.
Split into simpler sentences: After the ship sank, Rose and Jack clung onto a piece of driftwood for almost a half an hour. As there was only enough room on the wood for Rose, Jack kept most of his body immersed in the freezing cold water.