And when the laughing stopped the real reason emerged . . . . . .
The real reason why bananas are not carried on ships is that they exude a gas while ripenening that accelerates the ripening of other fruit and vegetables and if stored with your other perishables will substantially shorten their shelf life.
Many cruisers, myself included, have carried a bunch of bananas hung off the stern to ripen away from any other supplies and have never had any issues with the superstition surrounding them. In fact, it was only recently that I heard that this fundemental physical attribute was construed as a superstition. They're a superb food source.
Then again, while being superstitious isn't a requirement for safe sailing, it doesn't do any harm