Related to Space Madness. Being away at sea for a long time seems to take its toll on your mind. Maybe it's the unchanging landscape, maybe it's being away from your loved ones. Either way, in period pieces and even sometimes in modern ones, you can expect any characters away on sea for extended periods of time to go crazy. Historically a Truth in Television. In addition, there's a lot of validity to the "don't drink seawater" idea, since salt water has the nasty effect of making you even more dehydrated, which can lead to delirium. And, y'know, death. Compare Cabin Fever.