Make sure you are using single servo focus if you are locking with half press...
I would use Continuous Servo, and lock with AF Lock, if I used AF.
I would also turn the dynamic AF off- it will frustrate you.
One focus strategy for people who are comfortable with half pressing their camera through the housing is to us half press to start af, and lock when focus either locs on part of the subject (as James said), or gets in the ball park. From there, you have to achieve critical focus by moving the camera back and forth (while holding AE-L/AF-L).
Another strategy is to configure the AE-L/AF-L button in the menu to be AF-ON. The eliminates the prefocus function of the shutter release. The camera will only AF when you press it down. The advantage of this is that when you release the button, focus is locked (so you don't have to hold down AF-L while recomposing/fine focusing). This is also good for people who don't have the most nimble fingers.