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Phil, I did some searching for information that can help you last evening, but it appears that only tide tables are available in your part of the world. So I'll give you some general information to work from ... you'll need to rely on local knowledge to fill in the gaps. Most of what I know is based on diving conditions here in Puget Sound. You're almost on the opposite side of the planet, and so it's reasonable to expect that your conditions are in some ways similar in that you can get very large tidal exchanges that can seriously impact conditions at a given dive site.
From a diving perspective it's mostly current you have to worry about ... tides can impact visibility, and as someone already mentioned ebbs usually create worse visibility than floods. That isn't always the case, but at a lot of sites it tends to be true. Tides also create current, and that's what mostly affects us. How the tides affect current is very much dependent on how water flows past the site as the tides ebb and flow. Water flow can be affected by how the current sweeps along shore, points of land, jetties and other manmade structures, and bottom topography. Water tends to speed up along shallower areas, particularly if it's coming from deeper ones. Walls and dropoffs can create downcurrents ... underwater waterfalls, if you will. Constrictions between two landmasses will not only create more current, but can create eddies that send the current in directions other than you'd logically expect. At some sites, current is much more severely impacted on one side of the cycle than on the other. For example, we have a popular site here that can be dived pretty much any time on a flood, but you have to be very careful diving it on an ebb because of the way the water flow hits the site ... on a flood it's protected by a jetty and on an ebb it's not. Some sailing charts will show you how current flows around a given waterway on ebbs and flows, and this can give you some clues as to how (or whether) to dive a site on a given tide cycle.
Also keep in mind that slack current generally does not occur on a high or low tide ... water's very heavy stuff and it builds up tremendous inertia. For this reason, slack current tends to have a time delay ... usually but not always following a high or low tide by a certain amount of time. That time can be predicted within certain limits, but keep in mind that the volume of water flowing during a tide cycle will affect the time delay. Some sites are less predictable than others unless you understand how this works. For example, we have another site that is a passage between an island and a peninsula. Water flow there is much stronger during a flood than an ebb, and although common sense would tell you that diving it on a weak ebb would be OK it is not ... because on a weak ebb the inertia from the flood overpowers the incoming water of the ebb, and the water never stops flowing out ... away from shore. That particular site can only be dived on a moderate exchange, preferably around slack before ebb when the current will take you out during the last part of the flood, and then allow you to swim back in on the weaker ebb to return to the exit. Mistiming the slack, or diving on too weak an exchange will mean you either make landfall at a park and have a long hike back to the dive site or you get swept out into the straits ... in which case hopefully you brought your passport, because you're going to Canada.
Here in the Northwest planning our dives around tides and currents is necessary at most of our better dive sites. We rely on several resources ... books published by local divers, dive site reviews by people who have dived the site, current corrections in an XTides website for the local NOAA buoys ... with time corrections factored in based on local knowledge, or simply stopping in at the dive shop that's most local to the site and asking somebody. I don't know what resources are available in your area, nor do I know if South Africa has anything like NOAA that installs and monitors buoys to measure tides and currents in your local area. But I would also check at a local boating store to see what resources they have available ... sailboats in particular are almost as reliant on knowledge of local currents as divers are, and those resources can be helpful.
... Bob (Grateful Diver)
From a diving perspective it's mostly current you have to worry about ... tides can impact visibility, and as someone already mentioned ebbs usually create worse visibility than floods. That isn't always the case, but at a lot of sites it tends to be true. Tides also create current, and that's what mostly affects us. How the tides affect current is very much dependent on how water flows past the site as the tides ebb and flow. Water flow can be affected by how the current sweeps along shore, points of land, jetties and other manmade structures, and bottom topography. Water tends to speed up along shallower areas, particularly if it's coming from deeper ones. Walls and dropoffs can create downcurrents ... underwater waterfalls, if you will. Constrictions between two landmasses will not only create more current, but can create eddies that send the current in directions other than you'd logically expect. At some sites, current is much more severely impacted on one side of the cycle than on the other. For example, we have a popular site here that can be dived pretty much any time on a flood, but you have to be very careful diving it on an ebb because of the way the water flow hits the site ... on a flood it's protected by a jetty and on an ebb it's not. Some sailing charts will show you how current flows around a given waterway on ebbs and flows, and this can give you some clues as to how (or whether) to dive a site on a given tide cycle.
Also keep in mind that slack current generally does not occur on a high or low tide ... water's very heavy stuff and it builds up tremendous inertia. For this reason, slack current tends to have a time delay ... usually but not always following a high or low tide by a certain amount of time. That time can be predicted within certain limits, but keep in mind that the volume of water flowing during a tide cycle will affect the time delay. Some sites are less predictable than others unless you understand how this works. For example, we have another site that is a passage between an island and a peninsula. Water flow there is much stronger during a flood than an ebb, and although common sense would tell you that diving it on a weak ebb would be OK it is not ... because on a weak ebb the inertia from the flood overpowers the incoming water of the ebb, and the water never stops flowing out ... away from shore. That particular site can only be dived on a moderate exchange, preferably around slack before ebb when the current will take you out during the last part of the flood, and then allow you to swim back in on the weaker ebb to return to the exit. Mistiming the slack, or diving on too weak an exchange will mean you either make landfall at a park and have a long hike back to the dive site or you get swept out into the straits ... in which case hopefully you brought your passport, because you're going to Canada.
Here in the Northwest planning our dives around tides and currents is necessary at most of our better dive sites. We rely on several resources ... books published by local divers, dive site reviews by people who have dived the site, current corrections in an XTides website for the local NOAA buoys ... with time corrections factored in based on local knowledge, or simply stopping in at the dive shop that's most local to the site and asking somebody. I don't know what resources are available in your area, nor do I know if South Africa has anything like NOAA that installs and monitors buoys to measure tides and currents in your local area. But I would also check at a local boating store to see what resources they have available ... sailboats in particular are almost as reliant on knowledge of local currents as divers are, and those resources can be helpful.
... Bob (Grateful Diver)
