Ebb = The movement of tide back towards the sea. This exposes the intertidal zone (aka Falling Tide)
Flood = The movement of tide from the sea, towards land. This covers the intertidal zone (aka rising tide)
High Tide = The point where the sea changes from Flood to Ebb
Low Tide = The point where the sea change from Ebb to Flood
Slack Tide (Slack Water) = Period of time just before and after High and Low Tide.
As we get to about 1+ hour before High Tide, the flood slows dramatically. At and about 1+ hour after high tide, the ebb begins very slow. There is relatively little change between slack flood to slack ebb. The same occurs just prior to and following the low tide. The majority of water is moved during the exact mid point of the Ebb and Flood.
I'm not sure if you can calculate the actual speed the flow is travelling as that will vary based on the natural geography of the area. It will tell you whether or not you will be bucking the tide or running with it and whether you are running at the best or worst times. Lately, we've been playing around with running WITH the tides, and preferably when they are flowing the strongest. We've found a surprising amount of fuel economy in doing so.
We've also been looking at wind speed and direction. Wind opposing tide makes for a snotty ride. It also depends on if you're travelling through an odd area, like The Race that seperates Long Island Sound from Block Island Sound. There, crossing during a slack is much preferred.