The cruise was billed as a 4-day round-trip voyage out of Miami -- and Michaels promised to be "out and about" with the fans ... signing autographs, mingling and even guest bartending.
But Tuesday, fans received an official cancellation notice from the cruise company citing "lack of participation."
But Bret blames the whole thing on a "less-than-reputable promoter" -- who "feels he can take advantage of both artists and fans."
Michaels -- who is currently touring with Poison and Motley Crue -- explains, "I want to reassure everyone that I have went out of my way to try and make this the best fan cruise on the high seas ... Know that I am working hard to right this wrong."
Bret also ensures fans, "All money will be refunded to you."