Navigator Boy

In both films the main characters, Keaton's Rollo Treadway and Elliott's Nathanial Mayweather, are both clueless, horribly sheltered rich brats. Keaton's character isn't as obnoxious at Elliott's, but they are equally unprepared for dealing with the real world.

As the movies start, both characters intend to board a cruise ship destined for Hawaii, and accidently board the wrong ship because a sign is partially covered.

The big differences in the film are that Rollo boards a deserted ship which is cast adrift, with his unrequited love, an equally clueless sheltered rich kid. Nathanial boards a small fishing boat and isn't noticed until they're under way, and unwilling to turn back. His love interest doesn't show up until midway through the movie, and she's a tough ironworker.

The Navigator has an underwater diving scene (Buster Keaton actually went down in a diving suit for these shots!) where he has a sword fight with a swordfish. Cabin Boy has an underwater scene where Nathanial becomes friends with a shark/man.

Rollo tries to drink a cup of coffee made with sea water, while Nathanial enjoys a cup of chum, thinking it's bullion:

The Navigator has a scene with Rollo trying to pull a cruise ship with a tiny row boat, and Cabin Boy has a scene with Nathanial on a little raft towed behind the fishing boat.

A scene that appears later in Cabin Boy which is taken from The Navigator is one main character riding the other like a surf board. In The Navigator, Rollo is wearing a diving suit, which is blown up like a balloon, so he floats. In Cabin Boy, Nathanial rides his expert swimming girlfriend like he's water skiing.

The last third or so of the movies are more divergent. Rollo fights off cannibals and finally gets rescued by a submarine. Nathanial enounters some mythological figures, makes it home, rebels, and decides to return to a life at sea.