There is one on the southwest side of Islamorada (across the hwy from a marina) that feeds hundreds of tarpons on the gulf side beach and has a big saltwater pool filled with amazing fish.
That would be Robbie's Marina. It's famous for the hundreds of huge Tarpon that hang around the docks waiting to be hand-fed bait fish. As far as the saltwater pool, you may be thinking about the Theatre of the Sea, across the bridge and a few miles to the East. Robbies is open water--the Tarpon leave at night to hunt on their own.
Robbie's also is good for breakfast.
I was down there in December and stayed at La Siesta in Islamorada. It's been sold and is under a complete makeover to condos but seems to have stopped halfway. I got the impression that maintenance is a bit borderline due to impending demolition of the remaining hotel buildongs. The Islamaroda Fish Company is a good place to eat on the way down or back, schedule depending.
We have children so we're not much out barhopping (

:-/

) so we typically stay north (east?) of Key West. Since you will be able to take in things fun for grown-ups then you're on the right track to stay in KW and walk. For a bit of old Florida go find the No Name Pub on Big Pine Key. Turn off of US1 on Key Deer Boulevard on Big Pine Key then immediately right on No Name Key Road. It's been there since the 1930's. http://www.nonamepub.com/
Edit: You may already know this but it's highly adviseable to use the Fla. Turnpike instead if I-95. Get on the Turnpike about Stuart or Jupiter and stay until the end at Florida City. You don't want to go through all the construction and crazy drivers from West Palm to Miami. The tolls on the turnpike are far preferable, IMO.