Can i use a Narrow sea to reduce a character and if it have a poisoned well played again and reduce again? i believe not because de cost of discard location its not paid but im not sure.

If you use a Narrow Sea, discarding it is a cost. At that point, it's Moribund: Discard, so Poisoned Well will do nothing (since it's not a replacement effect).

If you're looking to use Poisoned Well to your benefit, you're better off putting it on a 0 cost reducer you can put back into play that phase (eg. Flea Bottom). It's typically not worth the card (poisoned well) to do it, though.