I assume you're typing this on your phone from the hot tub in question: good first step. Typing sullenly while everyone else gropes their way to regret is a great way to signal that you're less into this than they are. Just know that there's a very real risk a flung leg or wayward boob will knock said phone into the water.
If that does happen, DO NOT GO AFTER IT. If you think what's happening above water is unsightly, what's going on down below will haunt your nightmares. Okay, now that we've done the ground work, it's time for step two. And this is by far the most crucial of the steps, so pay attention.
2. Get out of the hot tub. Leave. Go inside.
Yes, it's as powerfully simple as it is sensically obvious. The best thing about a drunk person who's annoying you or creeping you out is that they likely won't care if you leave, or stay, or burst into flames. Especially one involved in some drunken friskiness.
Let them orgy. They can orgy all night and deal with the awkward silences tomorrow. You are a living, breathing human being, and that means you get to say what happens to your jiggly parts. Mayhap there is a less sexual hot tub down the way.
And remember, if all else fails, there's always the burka swimsuit. I hear those are a real drunken orgy killer.
I like that you used "orgy" as a verb.