The best version I ever heard of the song "Softly, As I Leave You" was played by a marching band in a Macy's parade. The band's utter incapacity to strive for poignance became in itself poignant. I got the same wacky pang when I heard Ethel Merman seize upon "What I Did for Love" on a special two-hour musical edition of "The Love Boat," to be seen at 9 tonight on Channel 7.

To say this is ABC's finest "Love Boat" ever is to lavish the faintest praise imaginable, but the tackily glittery program does weave a bizarre spell, listing as it does from sweetness to morbidity. In addition to Merman, the cast of indomitable old pros includes Ann Miller, Van Johnson, Cab Calloway, Carol Channing and Della Reese.

Reese is out of her league in this company, Calloway doesn't get to do enough singing, and Channing seems merely pathetic with her prehistoric kootchy-coo's. But Merman is like Patton; she's irrefutable, especially when she reprises her big hit from "Gypsy," "Everything's Coming Up Roses"--one of the most mesmerizing pick-me-ups in theater history --near the conclusion. She sings it to Johnson, who's playing a penniless ex-movie star hiding in his cabin and refusing to go on for the big Love Boat follies.

The follies themselves are the finale, but the performers sing and dance throughout the program, Miller getting off to a roaring start when she leaps onto a table and taps out Cole Porter's "It's All Right With Me." When the chorus boys tell her they love her, she shouts out, "How cute!," and later she remarks, "I just dance a little--you know, the way Luciano Pavarotti sings a little." Of all the elder stars assembled for this special show, none is a more awesome act of historic preservation.

"Love Boat" has always been something of a floating wax museum, and this one sets a new high in middle-class camp, but before anyone hoots at Merman looking grannyish as "Miss Acapulco" in the finale, or Miller's turn as, ahem, "Miss Panama Canal," it must be pointed out that the older performers are the ones who bring vitality and sparkle to the show. It's the newtimers who drag it down: The Love Bores, the program's insipid regulars, of whom only Ted Lange's genial bartender shows signs of real life (he has a high time singing "You Make Me Feel So Young" with Reese).

When members of the regular cast attempt to perform--attempt to entertain, as it were--that's when this "Love Boat" is saddest, especially in the cases of Lauren Tewes and the grisly Jill Whelan klutzing out a song and dance routine, or human thuds Fred Grandy and Bernie Kopell murdering Hope and Crosby's "Put 'er There, Pal," or cloddish Gavin MacLeod clomping around in sequined tails.

The world was such a gladder place when Hollywood made musicals (and not poison-pen letters like "Pennies From Heaven," either) and TV variety shows provided a home for old vaudeville stars--a link to crowd-pleasing traditions of more glorious and less prefabricated eras. In harkening back, however awkwardly, to that day, this "Love Boat" excursion proves a pure-pleasure cruise.