There’s one funny joke here. That’s it. And it lasts for about 20 seconds. It involves a grown man in the past calling his future wife (who is currently only nine) and profanely accusing her of cheating on him… 20 years from now.
The gag comes near the end of the movie, and by that time, it’s too little too late. Point in fact: Hot Tub Time Machine is as goddamned stupid as you’d expect a movie named Hot Tub Time Machine to be.