How big is the float going to be? The span really determines the size of the joist. Have you already bought the PT? Hemlock will be cheaper, lighter and it is pretty rot resistant because it has a good level of tannic acid in it, not as much as cedar but close and it will not leach harsh chemicals into the fishing enviroment. Check Parkers in Bradford or Yoders in Corinna or Crescent in Stillwater.