Hi Paul,
 
You'll have to wait until you get to heaven to ask the Lord why Adam and Eve were created naked.  However, if they weren't created naked and were already wearing clothes,  then the Lord wouldn't have had to clothe them with coats of the skins of innocent animals which could be considered a type of sacrifice for them because they had been disobedient to God and sinned.
 
As for the second part of your question as to why believers will have robes and not be naked, Rev. 19:7-8 tells us that wife of Christ is "arrayed in fine linen, clean and white: for the fine linen is the righteousness of the saints."  Since the Bible is the truth, we will have to wear what the Lord shows us that we will be wearing. If he said in Revelation that we all would be naked, then we all would be naked, but it doesn't say that.
 
As for wearing undies under our robes, the Bible doesn't tell us whether we will or we won't although the priests in the temple wore linen breeches to cover their nakedness according to Exodus 28:42.  So we will just have to wait and see.
 
As for swimming,  long robes aren't normally worn in the water because the weight of the wet robe could drag you down and cause you to drown.  It would be impossible to drown in a long robe while having an immortal body because you can't die. <G>  If Jesus could walk on water in a mortal body and walk right through a wall in a glorified, immortal body with a long robe on, you would have no problem swimming in water with a long robe on either.  <G>
 
God bless,
Donna