[Tuor of Gondolin]

While this might not be easy, an island(s) should be located,
somewhere in distance from the mainland between
Balar and Numenor, from which to conduct raids and be
a base of resistance, while also sowing dissension among
the bad guys over the ring. (Think England against the nazis
in 1940-1942). If it's known among the Easterlings, et. al.
that the Great and Powerful Sauron is impotent at sea and
in his inability to get his precious back all the better for the good guys,
who could have bases in mountain strongholds, islands, etc.

Hmm. How about putting Gollum on an island with his
precious, the location known only by Cirdan, Gandalf, etc.?
But with an agreement (mutually watching each other) not to land on the
island.