We had some good fish tacos at the McCormicks and Schmicks happy hour at the Omni Hotel next to Petco Park..
Tin Fish is decent...if they have a bike rental, take it to the Fish Market next to the USS Midway and have a couple at the Oyster Bar or go the other direction and hit up the Mariscos truck.
Just make sure that they use a Mexican crema vs. some crap ranch dressing.
Depends on how far you are willing to walk.
If you want to walk about 40 minutes each way, and aren't scared of poor/homeless people, you could go to the excellent Mariscos Beyer truck at Imperial and 22nd:
A cab ride there is also quite doable.
Some may suggest Tin Fish or Blue Water, but I wouldn't.
Also, note that the Hyatt does appear to have some kind of bicycle rental available, which could be a good way to go if you're into that sort of thing.