It's Dark Life by Kat Falls. What I liked the most about it was there aren't that many books out there where people attempt to colonize the seabed. I thought that Kat Falls did a great job coming up with all the ways it could happen. Just when the reader became skeptical about the science, she came up with a reasonably plausible explanation. Although the science teachers at my school did not buy the idea of the liquigen capsule taking care of the air pressure issue.
We ran some science class based book discussion groups last year and this one was the hit of the series. I wrote some questions for these parent-led discussion groups and have posted them as a free download at my store at Teachers Pay Teachers. Eleven meaty questions for free!
I've got the sequel, Rip Tide, on my list of reads. I'm wondering where she can go from here because I felt like she already had to reveal much of the surprise of living under the sea.