Oh dear, I would personally make any excuse necessary to get out of such a trip! LOL.
I recommend a one-piece, or at most, a tankini that is long enough it doesn't show any midriff. I would select the most conservative suit you can find (high neck, with padding for coverage, dark color), and pick a good cover-up to go with it - and wear the cover-up as much as possible. That way you're "fitting in" and not being a killjoy, but also keeping it modest.
The question you need to ask yourself is this: do I want the vision of myself in a bikini or with bathing suit cleavage hanging out, forever etched on the eyeballs of my colleagues?
You know, I was one of the people that said a hint of cleavage is okay in the office (especially since if you are larger chested, as it often can't be avoidable), so I am really not all that modest, but this is a different story altogether, in my opinion. I just don't want my colleagues having a picture in their mind of me wearing a bathing suit, period. Even a modest tank suit. I'm pretty picky about what I wear for workout gear at work too (we have an onsite gym).
I can remember one time my boss suggesting we have an off-site at the beach; we quickly nixed that idea. Then there were the office parties at the CEO's house - he had a pool. That was more workable because going in the pool was completely optional. The men all dived in; not a single woman brought her suit.
