This is a boiled shrimp recipe I got from someone who knows their stuff. After I got it I remember that my dad used to cook boiled shrimp just about the same way. 

2 1/2 gallons water
3 lbs. shrimp
Between 1/3 and 1/2 cup of salt
1/2 cup liquid seafood boil
1 tsp. granulated garlic
1 med. onion quartered
1 head garlic cut in half

In a large pot bring water to a boil. Add all ingredients except salt and boil about 10 minutes.
Add shrimp, stir and cover (be sure the fire is on high). Bring to a boil, uncover and time about 2 minutes stirring every so often. Here's the tricky part! Watch the shells on the shrimp. When it looks like the shells have moved away  from the shrimp meat (formed an air pocket) they're done. Smaller shrimp will cook faster than larger shrimp so don't put too much weight on the 2 minute time.

Once they're done you've got to cool them down quick to stop them from cooking. Once they're cooled just a little add the salt. after a few minutes start tasting. The longer they soak the more seasoning they'll soak up.

If you have a 2-litter bottle of frozen water you can stir the shrimp with it to cool it down.
You can add some ice cubes too but not too many.