An epidemic is loose in movieland, killing off all the mommies. This week alone, two such women were reported gone, in "If Ever I See You Again" and "Sea Gypsies," in the peak of youth, of unnamed causes.

What they leave behind, besides poor little motherless children, is young, vital but tender widowers, heroes of what seems to be a big film-fomula revival. These widowers are terrific at tucking the children in bed and sharing adventures and smart repartee with them, and seem to be madly attractive to unencumbered young women.

Whatever is getting these mommies is not known. Possibly the Dutch Elm Disease. It strikes swiftly, but while Mommy never recovers from it, the rest of the family is fully recovered in a remarkably short time. In each film, there is a one-second tribute to the departed, in which a small blond child mentions mommy and all shake their heads sadly before jumping in enthusiastically to Daddys new romance.

One suspects that these young women are victims of the rise of the un-macho male. A widow with children is hardly worth anything as a heroine, especially if she seems to which would seem unsexy; in a man, any nurturing at all is now considered to be terribly sexy. If Daddy doesn't even seem to notice that there is a panting young woman always in his path, so much the better.

In "The Sea Gypsies," Model Daddy has decided to take his two daughters sailing around the world, with no thought but to get to know them better. Some weeks later, when they're shipwrecked in Alaska, he takes notice of the young woman who's covering the trip for a magazine and sharing its work and disasters.

But the family aspect of this film is not badly done. Robert Logan talks to the children normally, not as if the caterer had sent them over and he wasn't quite sure what their duties were. The pity is that this slim formula is the only aspect of the story that's treated seriously, at the expense of dealing with aspects of nature, civilization and survival naturally suggested by their plight.Every now and then, someone in the group is chased by a bear. Quite often, as a matter of fact. That bear is one of the bigger bores around in films today.

That about does if for Nature. We're not shown how people survive without the tools of civilization. In fact, it's only by these tools that they are able to live. They seem to have remembered to pack a vast number of them - including one kid's water pistol, which has bear-routing properties - during the shipwreck.

Another opportunity is missed when the fathe rattempts to kill for food and then decides not to polish off a caribou because its about-to-be-orphaned little Bambi gives hom a melting look. Later he does kill a caribou, as well as some fish and animals with less personality. No point whatsoever is made from any discoveries they may have made about Nature's necessities when they got hungry, and what a luxury it is to be able to be compassionate.

The Daddy in "If Ever I See You Again" is a model father - or so his housekeeper tells us; he sure sounds phony when talking to his children - but his heart is on his own adolescent fantasy of impressing the girl he lost in college.

Daddy has no one but himself to blame for the fact that this film is so bad. Joe Brooks, who plays the lead, also produced, wrote and directed the picture and composed, arranged and conducted the score. Presumably he also selected the non-actor celebrities, George Plimpton and Jimmy Breslin, who play supporting roles, and "Charlie" cosmetic model Shelley Hack in her first big role, so he is also responsible for showing himself up as an actor with amateurs who are adequate in their parts.

The swet single-Daddy position is not the chief point of this film, but it is important because the nurturing aspect clashes so badly with the character's self-importance, professional swaggering (he is inordinately proud of having appeared in TV Guide) and the egoism he brings to his romance.

Childishness simply does not go with the job of being Superdaddy, no matter how many skating trips he takes the children on or how much credit he gives himself for having remembered to nag about the piano lesson.

It's interesting that the one good Daddy-and-children film this year, "Casey's Shadow," Mommy did not leave feet first. She walked out on her own two feet, and after you take a look around the household, it was with good reason. Perhaps this is what humanized Daddy.

IF EVER I SEE YOU AGAIN: Annandoe, Aspen Hill, Jennifer, K-B Georgetown Square, Londons Mall, Loural Cinema, Loehmann's Plaza, Marlow and Tyson's Cinema.

THE SEA GYPSIES - Alpine, Beocon Mall, Carousel Fairfax, Community, Crofton 1, Featherstons, Hampton Mall 2, Jerry Lewis 1, Londover 6, Manassas Drive-In, Mercodo, rolling Valley 1, Springfield Cinema 2, Town Center 1, Valley Mall 2, Village Mall, University, White Fint 5.