I am loving this. Loving it, loving it. A guy who knows his girl and cares about nuances, puts thought and heart into the planning instead of just logistics... oh, my!
The way you described her -- massage therapist, water lover, fish in a previous life -- then I saw that Etsy ring and the sky broke up and the angels sang. It's just SO fitting.
I don't know the answer to this, but some of the more seasoned board members may... are there any vendors who can recreate the Etsy setting at a reasonable price? I know they couldn't do it verbatim because then you're stealing someone else's design and that's not cool. But maybe they could do something similar that you would put your own spin on? I only see custom settings here that are more traditional in their "parts" and not as funky as this one.
You may want to look at www.greenlakejewelry.com
for inspiration, as well. They have some great organic style settings. I think they may be out of your price range, but they could serve as good guides for designing your own piece. Or maybe some of the designers there work in silver, for all I know.
Do a Google Images search for things like "organic ring setting" or "swirl engagement ring" or whatever keywords come to you. You can find some interesting jewelers via the images' links.
I vote that you not set a time you want to propose. Period. Keep the ring with you when you can (you probably don't want to leave it on the beach unattended while you swim), and when the moment is right, you'll know it. Maybe it will be traditional, like during a moonlit walk or over champagne at dinner. Or maybe there will be a moment of frustration (come on, we all know they happen) and ta-da, you'll turn the situation on its ear and change how that story will be told later. Leave your options wide open and "it" will find you.