Does the Pirate stop fish production when playing Seafarers with Fishermen of Catan?
In terms of Settlers expansion combinations, the catan official website really is a one-stop-shop. They have a combination listing for Seafarers and Fishermen, here. It says a few things about fishing and ships, but not about the pirate blocking fishing spots. Based on that alone, I'd say the pirate does not block fishing.

If that doesn't do it for you, this BGG thread about the same subject has a reply from a Kosmos representative (Sebastian Rapp), saying:


  Officially speaking: The pirate does not block fishing grounds. He just does what the rules say: He steals a resource and disables placing/moving ships at the borders of his hex.
  Everything else, like blocking harbors or blocking fishing grounds are variants, some officially mentioned, some not. So it is up to the players whether they want to play with such a variant or without.


As for your b) question, I'm pretty sure you can't place the robber on sea hexes. Since you can put boats on it, it's a sea hex.