An early and in many ways defining example of the 1970s disaster movie, benefiting from an irresistible gimmick: a freak tidal wave causes a ship to turn over completely in the water, so the remainder of the film, detailing the survivors' struggle upwards to freedom, is played on a series of amusing upside-down sets.
As the rules dictate, the first half is basically soap opera, introducing a disparate assortment of characters via irrelevant, soon to be forgotten subplots. (We have the usual mix here: a sweet elderly couple, a precocious kid, a no-nonsense cop, a young girl who can and does sing, even a pre-ironic Leslie Nielsen.) Then at the halfway point, disaster strikes, and most of the characters we haven't got to meet yet are killed in grand fashion. Part two details the remainder's attempt at survival, with much ingenuity, self-sacrifice and peril, through the course of which at least two of the characters we're rooting for will also die. The only odd note here is the decision to make Gene Hackman's hero a pretentious, bad-tempered vicar.
Hard to decide what the fundamental appeal of these films was: was the thrill merely one of prurience/sadism, a kind of dramatic ambulance chase, or, more nobly, of enactment, an exercise in preparedness and vicarious survival? Or perhaps it was purely technical, and down to the sheer elaborateness of the special effects carnage? If the latter, this one comes up a little short forty-some years on: unlike the spectacular canvas of Earthquake or The Towering Inferno, most of it is shot in enclosed interior sets, with little real indication that we are in the bowels of a ship. Stock and model shots fill out what little footage there is of the exterior: even at the end, as the surviviors are cut free and helicoptered to freedom, we are denied a long shot, reinforcing the feeling that there is no ship, and minimising the sense of awe that is integral to the formula's effect.
Beyond the Poseidon Adventure (1979) was an unlikely and belated sequel, in which more or less nothing happens.