round is an adjective to describe something that looks similar to a ball, it is also a slang term for the word around. I think you want to use the word around in this case, unless there is a specific reason you are using the slang term. If you are using the slang on purpose then it is most common to put an apostrophe (') in place of the sound you are cutting out. In this case because you are cutting out the "a" sound you would write: " 'round ."

It doesn't make sense to say it unless there's something that it is. So If there's already a restaurant then that's fine. Otherwise you'd want to have there if you're saying that there's a nonspecific restaurant.

You should say "there was" instead "it was" :)

Try: The guide told the tourists that there was a good seafood restaurant around the corner from the museum. You usually wouldn't use the word "it" in that context because it would be too ambiguous. What is "it"? The reader is not sure what you're referring to by saying "it". When you say specializing in seafood I assume you mean a seafood restaurant. Specializing in seafood also just feels awkward in that sentence. Lastly you would say "around the corner" instead of "round the corner". You'll find that many native English speakers may use the phrase "round the corner", but it is still grammatically incorrect.