If you're looking for authentic seafood street tacos in Mexico, look no further. Don't listen to any tourist magazine, taxi driver, or friend back home and take it from a diehard foodie and a girl who has a PhD in taco eating: THIS IS THE PLACE! Marisma Fish Taco is a gem and it's on my must-eat list every time I'm in Puerto Vallarta (which is about once a year).
The hours of this little taco cart vary, so don't be surprised if you first show up and they're closed. I find that going around lunchtime during the week gives you your best shot at catching them open for business. This is as local as it gets, and most of the workers swing by for a quick, satisfying and cheap lunch on the go. This place is hard to find because it's off the beaten path and is far down a small side street. Have patience and you'll eventually stumble upon it. Look for their orange and blue signage and you'll know you have arrived.
The taco cart has limited seating on wooden stools so you may have to wait for a seat. Belly up to the counter and place your order. You pay after you're done eating. Bring pesos for the best deal. This is one of the cheapest and tastiest meals you'll have in Mexico. Trust me.
I am partial to the grilled shrimp quesadillas, made in the traditional way which is basically like a taco with melted cheese inside, not the Americanized version that has two tortillas with the stuffing in between. The corn tortillas are made to order right in front of you and they are incredibly delicious. I could eat them by the dozen.
Homemade salsas are available for free (watch out because some of them will light your mouth on fire!) and tasty. The menu is straightforward and small, and they excel at seafood tacos of all kinds.
One word of caution: this isn't the place for food snobs or for people who aren't adventurous eaters. This is straight up basic street food (with a bucket and hand sanitizer for hand washing when the small outdoor sink doesn't work) and you literally sit in almost in the middle of a busy street!
Pro tip: there's a convenience store on the corner nearby. Pop in before dining and buy a cheap can of beer to drink with your tacos.