Can anybody recommend a hotel wit a  good/great seafood restaurant and, ideally, with no casino.

You could stay at the Delano that does not have a casino and it is right next to Mandalay Bay.
However the restaurant I am going to suggest is actually on the mandalay bay so you would have to walk through the casino to get to it.RM seafood very good I had the swordfish and some sushi and one other fish I can't recall now.
I can't think of any hotels that don't have a casino but a great seafood restaurant.
Perhaps others will have suggestions.

Some are off strip and some are on. Let me know if you want company for dinner!

Definitely try Aqua Knox or Fin

I always include at least one visit to Emeril Lagasse's Fish House in the MGM Grand whenever I am in Las Vegas.

Just had dinner there last week and it was fabulous. Excellent food, terrific service and if you can get a table out on the patio you will have great view of the lake of dreams.

Mastros Ocean Club inside the Crystals Shopping Center at City Center.