In an effort to NOT repeat myself from the thread that Hozer linked (Search IS your friend), I will answer the question about prior swimming experience. When I went through RSS (July-August 1983), I had not been a competitive swimmer nor been on a swim team nor anything remotely related. I was just a kid who liked the water and could hold my own. I did not join the Navy to be a Rescue Swimmer. I joined to be an Aircrewman. Indeed, I was FORCED to become a swimmer, as coming out of AW 'A' School I was awarded my second choice in platform, HSL (back then, one of your choices in platform HAD to be Helos if you were found to have decent (20/40?) vision and good depth perception). Also, when I went through RSS (run by HC-1 at NASNI vice the Aviation Schools Command at NASP), DOR was not an option. If you failed out, you were then sent to the Fleet as an undesignated Airman. A fate worse-than-death to those of us who wanted to fly. Especially as it usually meant you were sent to an aircraft carrier as ship's company to chock-and-chain airplanes.
With all that being said, I was able to hack RSS without a deep swimming (see what I did there) background. I don't think anybody in my RSS class had a swimming background. That school is more about mental toughness anyway as your preceding time at NACCS gets you into shape physically. Like most anecdotes about Navy training you will find on this site, a good attitude and mental toughness is all you really need to succeed.
Good luck.
