We have had very good Pacific lobster at Los Arbolitos and at Las Casuelas in P.V.
Overall the best seafood in P.V. is at "Mariscos Polo", which is not fancy but has very well prepared extremely fresh seafood. Polo, the owner, does have lobster sometimes but will only serve it when it is at its very best. If lobster is on the menu there, you cannot go wrong.The Pulpo burritos are incredible and are almost always available if you like octopus but the shrimp ones are amazing too.
Art is correct in saying that Pacific lobster is inferior to Main lobster, however the Mexican presentation and preparation at Los Arbolitos and at Las Casuelas is quite good. Remember that Pacific lobster has no claws. So you lose a lot of the highest quality meat because of that as well. Los Arbolitos is a little nicer after dark because the restaurant takes on a festive Mexican look at night with the lighting and the view of the hills, river, etc. Las Casuelas is only open at night I believe.
If you go to Yelapa, the little grass shack hotel that is there has good lobster. If you go to Sayulita, Don Pedros often has excellent lobster on the menu as well.
I highly suggest when y ou are in Cabo to go to the Trailer Park, just ask any taxi to take you there, it is about a five minute drive. They have not only outstanding lobster, but the best deep fried red snapper you can every have anywhere. The scallops are golf ball size. It's a really neat restaurant, outdoor. The best by far. Not to be missed.
My favorite place in Mazatlan is Tony´s on the beach (between Caravelle and Fiesta Inn Hotels). People confuse another couple of rest, called Tonys also. This one is on the beach. Lobster and Jumbo shrimp platter---2 lobsters and 6 jumbo shrimp (for 2) with a tortilla soup or green salad (each) and one drink each (Pacifico, margarita, etc.) for $350 pesos. It is a lot of food and also it is excellent!