Maybe they were drunk. If you're lucky they won't even remember or will be embarrassed and act like this never happened. People can take a joke too far when they are drinking, I find the suggestion they'd pick your skin off bizzare and frightening. And in no way does suggesting throwing her son in make it ok for them to gang up on you this way. Sadly, people don't always behave very well. That said, I don't think it's good to talk about or actually throw someone in unless it is a giggling child who wants it and knows how to swim.
See what happens. If they act like nothing went wrong, maybe go along.