The Deep By Gordon Korman

The Deep is the second book in Gordon's third smash-hit adventure series, the Dive trilogy.

In book one, we met four teens who were accepted on an internship with the prestigious Poseidon Oceanographic Institute, an internship program that consists mainly of utterly ignoring them.

Concluding that they were selected not for their skills, but for their lack of them, our dive team comes to the conclusion that their Poseidon superiors are up to no good. Now the kids are determined to discover the lost pirate ship, sunk in this area, and find any possible treasures before the Poseidon team can.

Unfortunately, there are many dangers in the waters, human and natural, and some of them may be more than the inexperienced team can handle...
 
- Goodreads Summary