"The Love Boat" was composed by Paul Williams and Charles Fox, and was sung by the ultra-smooth Jack Jones. From 1985 onward, Dionne Warwick sang the theme. Here is the Jack Jones era version, along with the opening credits sequence (talk about a potpourri of guest stars - Mike Connors, Elke Sommer and Halston?):
I know you'll be singing this to yourself for the rest of the day now (as I will be - loudly and proudly), so you might as well know the actual lyrics:
Come aboard, we're expecting you
Love, life's sweetest reward
Let it flow, it floats back to you
The Love Boat soon will be making another run
The Love Boat promises something for everyone
Set a course for adventure
Your mind on a new romance
And love won't hurt anymore
It's an open smile on a friendly shore
Welcome aboard it's love...
The theme would be used for comedic purposes (how else could it be used?) in Airplane II: The Sequel (1982), with Jack Jones making a cameo appearance. More recently, it was parodied in the South Park episode "Red Hot Catholic Love." Via the South Park site, here is a little bit of their version.