Road trip from MIA to KW, 23-26 MAR 2013, looking for suggestions of places for tshirts. But if it's a bar or resturant, I have to eat or have a drink there.

Any of the bars at the Key West Bight - the Half Shell, Turtle Kraals, Schooner Wharf, Conch Republic, e.g. have small booths or even a room with T's and caps and tanks which you can purchase without having to buy anything else there, but having a drink and shrimp or oysters is a bonus.
Even the Key West Bait & Tackle shop, across the parking lo t from the Half Shell has souvenir clothing.
Pjk
Oh, you must stop at Sparky's Landing in Key Colony Beach! Great food, beautiful setting, fabulous staff and very cool t shirts!
And once you're through with bars, there are ship-logo'ed ones - Fury, Danger, Sebago, even one for the Tortugas.
Pjk
Gotta get one from Robbie's in Islamorada!:)
In addition to the fine recommendations above...
Lazy Days - (MM 79 Oceanside, Islamorada) - I don't know much about tshirts there, but the food and ambience are top shelf.
Mangrove Mamas - (MM 21 Bayside, Sugarloaf) - great food, great vibe, and cool looking tshirts.
I'm guessing you could get cool shirts in the gift shop at Curry Hammock State Park in Marathon, the Keys Wild Bird Center in Tavernier, or the Eco-Discovery Center in Key West.
Happy travels, and happy shirt hunting. Let us know what you picked up!
Edited: 2:06 pm, March 08, 2013Stop at the old 7 mile bridge. Get a Save Old Seven shirt. Or hat, or sweatshirt.
You can't get them anywhere but at the bridge.
And they're helping a great cause.
Alabama Jacks (Key Largo) and No Name Pub (Big Pine Key).
AJ's doesn't have a website anymore (doesn't need one) but they have great Ts
www.yelp.com/biz/alabama-jacks-key-largo
Both are must have T shirts from the drive (and must stop bars!!!!)
I love my mermaid t-shirt from the Key West Ice Cream Factory, which also has fantasic ice cream and sweets.
Dont forget the Caribbean Club.