Wow, Snow! I'm impressed! The site is very pretty, informative, and thought-inducing. I never thought about all of the elements but water having a Summon. I think leaving water out may have been intentional in the game, though. Like Tidus is represented by the sun, Yuna is represented by the moon in Final Fantasy X. The items you need to power up her ultimate weapon are called the Moon Crest and Moon Sigil. The gravitational pull of the moon affects water (making waves), and with Yuna walking on water, rising on water during a Sending, etc, I've always associated Yuna with water. Nevermind that Spira is at least 80% water, as everything seems to be islands surrounded by oceans.
I love that you touched on the theme of sacrifice. It's super important in Final Fantasy X, and you did a wonderful job covering it. I found some typos on your site, so you might want to do a spell check. I can help you find them if you want, though. Just let me know!
Anyway, if you're interested, you can affiliate with Poltergeist! I'd love to affiliate with your site; it's super unique, and I think my site's visitors would really enjoy it.
Also, I made you a button, just because. I know this site was super personal for you with the attachment you had to your uncle, and I just wanted to do something nice to say congrats on finishing it.

Less tl;dr: I love this site!