The chart tells you when high or low tide occurs. So lets say 3:00pm is high tide. Then the incoming tide is going to start to slow down significantly just before 3:00 pm. Then it will slack at 3:00 pm (not move). Then it will reverse and start to moving in reverse direction by 3:15-3:20pm, this is not a hard fast rule. Sometimes it turns quick, sometimes it turns slow. If you have not been diving at the bridge before, a good rule of thumb is to enter the water approximately an hour before high tide, and exit approximately an hour afterwards. This is not written in stone. I see people entering 1.5 hours before high tide, and I typically might not exit until an 1.5 hours after high tide. Also if I get to there really early I might snorkel the trail before I even dive, say two hours before high tide. The idea of the hour before and hour after is the best visibility without to much current. Also if you google Blue Heron Bridge Dive, there are many good tip sheets, e.g.
Nine Things You Need to Know Before Diving Blue Heron Bridge - Coleman Concierge .
If you have not been there before, its a relatively large area, that can be thought of as three areas, the eastside span, westside span, and snorkel trail in front of the beach. It would be very difficult to cover the whole area in one dive. Best to pick one side or the other, or just do the snorkel trail. Better yet, you can a hire a guide for a more local perspective. Its not necessary, but there are good guides available.
@Scuba_Jenny does guides and she is on this thread all the time. Pura Vida divers and Force E also have guides for hire.
Oh and one or two other very important things, remember to use a frog kick, as opposed to a flutter kick. Flutter kick stirs up the bottom and reduces visibility for everybody. Be really aware of your buoyancy, its a shallow site so buoyancy control is somewhat more difficult than say on a sixty foot reef. Try to avoid making contact with the bottom or again it stirs everything up and reduces visibility, not to mention iddy biddy critters that might suffer damage from a human a thousand times their size. Good luck have some great dives!
