In the final analysis it isn’t a matter of whether you use rhetoric, but how. Inasmuch as your preaching is still public speaking...you’re going to get rhetorical.
A solid structure, a lively skeleton, inarguably makes your messages more life-giving. They will be clearer, more interesting, and easier both to follow and to remember.
The castaway senses he needs something more. And what he needs more, much more, than mere help with acclimating himself to life on the island is a message which transcends the island.