Most everything I have read says "be at the waters edge 30 minutes before high tide." With that in mind, when exactly should you ENTER the water?
This depends on several things.
There are some areas with far less current during tide change than others....
I can go in 2 to 2.5 hours early, but the current will be big when you go west of the beach ....If you like big current, as I do, not big deal. Most do not like currents over 2 mph, so if you want in early, you "could" walk east down the beach to 50 feet or so east of the shipwreck site ( between the cabin cruiser and the sailboat), and swim out there....current should not be terrible even 2 hours early, but there will be a current..stay low to the bottom. stay flat horizontal, and do frog kick so you dont silt up the bottom. If you dont know how to do frog kick, you need more instruction

From 2 hours early, once at the shipwreck, chill out there for an hour, then drift with the current towards the west and make the curve toward the bridge and channel crossing. The 18 foot deep area lets you out of the current a bit, and will be pristine still, as few lame divers will have been out silting up the bottom yet (by swimming head up and feet down, kangarooing along the bottom). Since this type of diver has zero skills, they have to wait until the current slows to a near stop, or they are blown to wherever the current wants to push them
There are also plenty of good divers who just don't like fighting currents, thus the wait til around 30 minutes prior to peak high tide. But this is not the only way to dive the BHB. Sandra and I can get 3 hours on a tank here any day we want to.