I'll post my experiences with both.
I took salsa dancing at a few different points in my life. Here is what I've learned....
With salsa dancing, the classes are essentially a loss leader for getting good enough at dancing to have a fighting chance to pick up in a real life venue with salsa dancing. The classes themselves usually won't have the most attrative women or even the youngest women unless it's a semi-unique situation with classes. If you're in college and taking salsa classes as a college extracurricular, you have a fighting chance of it being useful. Most often, it isn't. Also, you're looking at a long time in salsa classes to get your skill set up to the point where you might be in a real life salsa club. Additionally, if you aren't Hispanic/Latino, you'll also want to be fluent or nearly fluent in Spanish to complement the salsa skills. Learning Spanish also takes time if you don't already know it.
Salsa is a real time sink.
As for yoga, I've been in yoga classes a few times when I was at an extreme lack of prospects at a moment. I prefer other styles of fitness classes more, which also have favorable male-female ratios. Yoga women are a bit kooky if they are serious about it. Even with fitness classes having favorable ratios, getting dates from attending them isn't as easy as you might think. There are very narrow windows to approach. You have 5 mins before class and 5 mins after class. That's not much for quantity. Additionally, a lot of women are not very sociable before/after classes. That's true both for men and with other women in terms of female friendships. I have arranged dates at fitness classes but relative to the number of fitness classes that I attended over time, it was an inefficient time sink. The primary reason to go to fitness classes is if you enjoy the exercise style of the class. If you get any dates out of it, consider it a bonus. You can't build a pipeline around fitness classes. You'll likely have to supplement with other approaching venues.