I replaced the cheap outside speakers on our trailer with these
PolyPlanar MA-905 marine speakers. They have mylar cones with rubber surrounds, not foam.
I mounted these so that the woofer sat in the hole from the factory speaker and they fit perfectly. Decent frequency response (55hz on the low end) and plenty of capacity. I added a Kenwood 200W power amp with speaker-level inputs after the factory head unit and you could hear these halfway down the beach!
You can get them for a good price on
Amazon.