ABOARD THE MV MARITOL—Most tech startups struggle to stay afloat. Now a few are being forced into open water.

This 135-foot Norwegian ferry, docked at San Francisco's Pier 50, provides workspace for entrepreneurs. "The ship is kind of magical because it has this social cachet," says Jeremy Guillory, 32 years old, as he emailed would-be investors from a deck chair on a recent day.

On...