It was 20 years ago when I went through Giardasis, I'm 53 now - I think I did take flagyl. It was one of the most awful health things I've been through, and 6 months after it was over it came back. I had awful diahhrea and stomach cramps, I don't remember what other symptoms. I got it from going to a restaurant near my house - the next week there was an article in the small town paper that the water district up the mountain (where I live) had Giardasis. I'm guessing that not everyone who drank the water got sick, just those of us with lowered immune systems.
For me, mucus in the stool is a Candida yeast problem. That causes diahrrea too. Have you looked into that? Do you eat sugar? Yeast is a fungus that lives in your gut and makes you crave all the foods it lives on (like sugar and yeast).
Have you tried Dr. Schulze's Intestinal Formulas? I've used IF#1 for over 3 years, and use IF#2 when I do colon or liver/gallbladder or kidney cleanses. Dr. Schulze (800herbdoc.com) says that if your problem is diahrrea all the time, use Intestinal Formuna #2 alone. It will pull all the bad stuff out of your colon and stop the diahrrea.
You might want to get IF#2 and try it, and may be you'll be able to enjoy the cruise!