Look, can I say?:
"I'm skeptic that you can swim{\abot your ability to swim???}...."
"I doubt that you can swim..."
Are both possible?If yes what is the difference? and regard to the sent. in the curly brackets please and tell me what is the right form...
When you have a linking verb in this case (I am...), an adjective follows so it would be skeptical.

Both sentences are fine (the 'doubt' seems stronger to me, though).

Re: the curly brackets, either seems fine to me.