a few companies make marine speakers, you could slap those puppies in there.

if you live at the beach, then id say marine speakers are a must. otherwise get something cheap seeing as they wont last forever. nothing will last outside as long as inside, but mounted in the ceiling isnt really a "harsh" environment, so you could get away with normal in wall speakers...

my brand(s) of choice is polk audio (good value and good sound) or b&w (bad value and good sound). i have used and demoed polk audio's ceiling speakers and they sound pretty much like their other speakers "IF" they are boxed in as recommended by polk...