I find that I have to manually balance my AT Pro in salt water. This is especially true, if you've upsized your coil. The greater the coil size, the harder it is for it to balance.
The good news is that the manual balance feature is VERY easy to use (and often is faster than auto, anyway). There are two tones that happen with the manual balance, a high and low. The goal is to move the GB # lower until the tone changes (indicating you've gone too far), and then put it back up, until it stabilizes.
Basically, once you know where the balance is in the water, you can just set it there and do a quick check. GB for me was never an automatic event with my 14.5" coil on the beach. Had to learn to manual balance it.
Cheers,
Skippy