Deep water / Watt Key.

Enhanced descriptions from Syndetics:

From Watt Key, the author of the acclaimed Alabama Moon , comes a thrilling middle grade survival story about a scuba dive gone wrong and two enemies who must unite to survive.

It's the most important rule of scuba diving: If you don't feel right, don't go down.

So after her father falls ill, twelve-year-old Julie Sims must take over and lead two of his clients on a dive miles off the coast of Alabama while her father stays behind in the boat. When the clients, a reckless boy Julie's age and his equally foolhardy father, disregard Julie's instructions during the dive, she quickly realizes she's in over her head.

And once she surfaces, things only get worse: One of the clients is in serious condition, and their dive boat has vanished--along with Julie's father, the only person who knows their whereabouts. It's only a matter of time before they die of hypothermia, unless they become shark bait first. Though Julie may not like her clients, it's up to her to save them all.