Hi Scott,
I'm not a medator, but I play one on TV. I think that single titles like that would fall into trademark rather than copyright.
Because you have been using it for a while, that's your trademark. You could even use the TM symbol after the phrase, since you can claim it as your service mark. This won't prevent others from using it, but it might make a statement that you might take legal action in the event its used by someone else (although you probably never would). If you register it, then you get to use the R.
Its nice that this person asked permission, but the problem with giving away such a nice catch phrase is that others will see it and take it on as their own idea. A perfect example is the phrase that way too many resorts use "Come as a guest, leave as a friend". I've seen that at on at least six or seven websites, and it loses its effectiveness.
Perhaps you can give them permission to use is as a quote...
"One World, One Ocean, Let's Protect It!"
Scott Nielsen
If there are any lawyers who want to jump in and correct me, please do.
Scott, see you in Cozumel in a couple of weeks!
Bonnie