Complex sentence: Rafts were constructed, but several were swamped in attempts to carry a rope to the shore and the men on the raft that remained upright were forced to cut the rope to prevent them from foundering in the heavy seas.
Split into simpler sentences: Rafts were constructed, but several were swamped in attempts to carry a rope to the shore. The men on the one raft that remained upright were forced to cut the rope to prevent them from foundering in the heavy seas.