If you're just doing the 61.75 based on private you don't need the TSA background check. I hate to use the word logic and TSA in the same sentence but their idea is since you are not adding a skills set (no training is required) there is no reason to do the background check. Same as if you go from a private to a commercial. I can't find anything on the TSA web site but the AOPA has put out a guide covering TSA and they say TSA has "clarified" that no background check is required.
Since you say you want to do seaplane training I believe malc4d is right. The seaplane checkride will result in a stand alone FAA certificate. You can't mix and match between your based on and stand alone certificates. Since you are getting training to add a skill you'll need the TSA check.