Theory #1 - The woman has just waxed the kitchen floor, and just as it rains whenever you wash the car, a thundercloud has taken up residence over her house, casting an obvious shadow on the roof. This is funny.
Theory #2 - Waxed things are slippery. This is funny.
Theory #3 - The wax on her kitchen floor is so slippery that, after she spilled a pot of soup on the floor, the liquid was repelled onto the ceiling. Hence, the damp spot on the roof. This is funny.
Theory #1 - The gentleman is a paraplegic. He has misplaced his special buoyant prosthetics made from a rigid, closed-cell foam with a watertight rubber skin, or his "sea legs". He is forced to make due with his ordinary plastic prosthetic legs. The two women have thrown his floating legs overboard. This is funny.
Theory #2 - Women enjoy seeing any man suffer. This is funny.
Theory #3 - The nauseated gentleman is apparently searching the deck of the ship for his "sea legs", despite the rather obvious fact that they are clearly still attached to his body. This is funny.
Theory #4 - The "man" crawling around the deck of the ship is actually Katharine Cook Briggs, half of the team who developed the Myers-Briggs Type Indicator questionnaire. Her partner and daughter, Isabel Briggs-Meyers would be surprised that her mother had chosen to drink while aboard a ship: something an "ISFJ" would never do. This is funny.
Theory #1 - The man's digestive system is intolerant of the spices used in Spanish rice. The adult diapers he keeps on hand are "Tango" brand adult incontinence undergarments. His wife's advice to "put on some tangos" indicates her understanding that he will have a bout of inflammatory bowel syndrome tonight. This is funny.
Theory #2 - Tango is a dance mistakenly attributed to Spain, although it actually originated in Argentina and Uruguay. Spanish rice is a totally unknown dish in either Spain or Mexico. This is funny.
Theory #3 - A Tango is a type of dance, not a type of music. Theoretically, one could tango to any music with a beat. So, the woman's request to "put on some tangos" must indicate that the disc in the man's hand is a laser video disc, since a Tango can only be observed and not heard. This cartoon ran in the September 28, 1957 issue of The Saturday Evening Post, nearly thirty years before LaserDisc technology was made available to the American public. It is impossible that the couple owned a LaserDisc player in 1957, or that Bob Barnes, the cartoonist, had even heard of such a thing. This is funny.
[Commenter theories will be added to the post. -Mgmt.]