The low tide line is just that. If you're standing on the sand, you're above the low tide line.
If you're standing in the water, you're beneath the low tide line. Keep your feet and coil out of the water and you're OK.
As far as when you can and cannot detect in the water: I'm not sure on the finer points of that. I do know that in state parks, you have to stay between the tide line and the toe of the dunes. Also, a lot of areas in SE Florida have active salvor leases and you can't detect in the water regardless.
Detecting the wet has its benefits, but don't overlook the dry and intermediate wet along the flotsam. Honestly, you can make a trip out of just detecting the dry and still leave a ton of ground uncovered. The only time I'm really, really insistent on sweeping the low tide line wet is after a big storm.