My Platonic ideal of a fish taco is fresh fish, simply grilled (not breaded or fried), with some salad and lime juice and maybe salsa, but no crema or other dairy-based sauce.
When visiting Tulum (a few times 10 years ago, and again in 2017), my favorite fish tacos were at the cheap beach hotel Los Arrecifes:
The setting is decidedly low-key– it often feels almost abandoned, without much signage and with one employee (if you’re lucky) working in a kitchen, and a patch of sand covered in campers’ tents… but the tacos were fresh and simple– very different from any number of bars along the Tulum beach that advertise “Tulum’s best fish taco” (but whose focuses are just as much the bar, music, and a place to hang out).