could swim well. ---- could is the past tense of can , knowing how to do something . eg He could swim well last year but he can not do it well now .
was able to swim well. --- able is the ABILITY TO DO something , it means < in the past , he has the ability to swim well
eg He was able ( to have the ablility ) to swim across the river because he could ( know how to ) swim well
I dont think you are not able to finish this beer because you cannot dink beer fast .
I can do it because im able = im able because i can , can you see the difference