Quote Originally Posted by Throbert McGee View Post
I would tend to guess that shrimp are treated as a "mass noun" (like изюм, "a pile of raisins"), rather than a "count noun" (like арбуз, "a watermelon"). And from Googling, it seems that LIVE shrimp are "count," but COOKED shrimp are "mass". Thus, in the context of eating in a seafood restaurant:
No. That’s true for рыба, but not for креветка.
Креветки are usually plural. They can be either animate or inanimate — both variants are OK.

Quote Originally Posted by Throbert McGee View Post
В ресторане я покушал креветку.
--- At the restaurant, I ate (a big pile of many) shrimp. (NOT "exactly one individual shrimp").
Incorrect. Креветку here can mean ‘a single shrimp’ only.
Should be «В ресторане я покушал креветки» (inanim. pl. acc) or «…креветок» (anim. pl. acc. or pl. gen.)

Quote Originally Posted by Throbert McGee View Post
В ресторане я покушал креветки, потом попросил у официанта, "Можно «doggy bag», чтобы взять остатки с собой?"
--- At the restaurant, I ate some (of the big pile of many) shrimp, then asked the waiter, "Can I have a «doggy bag» to take the leftovers with me?"
Actually, this sentence doesn’t differ from the previous one.

Quote Originally Posted by Throbert McGee View Post
На море я пытался ловить креветок, а в конце концов я поймал лишь одну креветку.
--- At the seashore I tried to catch shrimp (anim. acc. pl.), but in the end, I caught only one shrimp (acc. sg.).
OK here.