I own both Seas of Blood and The Classic Play book. I must say that I am glad I have the Classic Play because of the hard bound nature and whatnot, but I haven't noticed much in differences rules wise between the two. Sure, they added on a few things here and there, but that wasn't necessary to sell me on it anyway. I'm a munchkin when it comes to Mongoose and a few other companies. If you have Seas of Blood, it wouldn't affect you... However, seeing that aftermarket sales are relatively high on Seas of Blood (some places I have seen twice cover price and rariety drives it up a bit too), you might want to wrap that up in a protective case and purchase Classic Play to use in a game...
Just a thought...