Big Paws Only Dog Forums

We have a 14 month pittbull, Casper.  He is very social with people and other dogs.  He has lots of energy, and loves both indoors and outdoors.  We want to get a second dog.  Our number one priority is picking a breed (or mixed breed) that will have a better likelyhood of meshing well with Casper.  I am looking for a dog that is social with other dogs, and loves water.  We really like large to extra large dogs as well.  Any suggestions?