Tung oil is great. I have used it a lot. It helps bring out the grain of the wood without altering the colour of the wood too much.
But, then I finish it with spar varnish,
or spar urethane. A couple three coats. Both should have UV protecting qualities.
If you plan on splashing the kettle around in the salt water surf …. Well, then I can't add anything more to this …