Alysser, in my opinion it looks as if you are afraid of being dumped and so you prefer to be the first to dump. This is a pattern that has happened before - I would call it the intelligent girl's way to trap yourself.
The solution is easy to describe (you know I could easily be your granny) and close to impossible to do: Just relax about yourself and about all that relationship thing. Being engaged at 18 is not important. What is important is to enjoy time with friends, women and men. And I am fairly sure you ARE ok - your posts sound like it.
So if you see that guy and go kayaking with him, why don't you enjoy it and try not to make plans. Guys usually like girls with whom they can have relaxed fun-and it sounds as if being with this guy would be fun for you as well. Just enjoy the moments and manage the expectations
I keep my fingers crossed for you.