This could absolutely be made into a feature-length game. Many people, myself included, enjoyed the cell stage of Spore very much. This plays like a much more in-depth version of that (and I'm all for it).
Additionally, in the earlier stages, my character uncannily started to look like a real little water creature, hopping around and looking for things to eat.
The dash was quite fun, but at the higher levels of upgrading it, dashing was 99% of my movement, and the base movement speed upgrades stopped seeming necessary.
The music was quiet but rather calm, and it fit the atmosphere quite well.
I like the idea of being in tunnels, it adds a lot more depth to the game (pun not intended) than just free floating through the water would. That being said, without a mini-map and with so many dead ends, I found it mildly frustrating sometimes to navigate. This stopped being an issue once I had my dash leveled to about 5 or 6.
The SFX and VFX were done quite well, too. Evolving was a genuine "dopamine" moment.
Fantastic job.
PS
Your use of the theme is top-notch. More importantly, it was well executed, too.