When the Ring was lying on the bottom of Anduin it couldn't attract Sauron because Sauron was still recovering from his loss in the end of the Second Age. He wasn't in his full power yet, and he didn't want to risk exposing himself by sending search parties. Anyway, he didn't know that the Ring was still in existence and he might thought that the strange feeling of an object of power nearby was because of something else.
I don't know about Frodo's and Sam's relative power.. they were both hobbits, and I don't think that their relative difference of power was so great. I believe that Frodo was detected so easily on Amon Hen because he wore the Ring on the Seat of Seeing. It's easy for me to imagine that while the Seat enhanced the vision of the one sitting on it, it also made him easier to see by other "magical" means, such as Sauron's palantir. Note also that Gandalf was aware of Frodo's presence on the Seat as well - he was able to guide him to take off the Ring (could this telepathic communication have something to do with the link between Narya and the One?).
If by some means Gandalf or Aragorn could have carried the Ring to Mordor without giving in to temptation, I don't think that they would have been more easily detected than Sauron. If they actually wore the ring, they would probably have been spotted sooner than Frodo or Sam, though. However, the weak points of that plan would have been that 1. Frodo wouldn't have given up the Ring to either of them under any circumstances and 2. both Gandalf and Aragorn would probably have been mastered by the Ring earlier than Frodo - their temptation to use it was greater because of their greater powers, abilities and desires to do good.
|