Ruby Redfort Take Your Last Breath by Lauren Child

All at sea? Ok, here's that lowdown...
 
Ruby Redfort: secret agent, thirteen-year-old kid. Super-smart, super-cool and not afraid of the water or anything in it. Sharks? Cut-throat pirates?
 
A giant tentacled sea monster? No problem, buster. But... hold your breath for more than 61 seconds? Yikes! Diving without oxygen is not Ruby's strong suit. 
 
Can she do it? There's only one way to find out...
 
Suitable for ages 12+