We often get asked "when is the best time to go snorkeling ?" Answering this question is tricky, the sea can change so much in such a short period of time that we spend many hours trying to predict the "perfect conditions" - even though getting it "perfect" does rely on some luck, there are several factors that you should consider
(These "rules of thumb" would apply to most rocky shore / inter tidal snorkeling spots in KwaZulu-Natal and not necessarily elsewhere in the world)
Not all tides are alike. A spring low one month is NOT the same as a spring low the next. We have Spring Tides twice a month (a few days around full and new moon) - generally speaking, spring low tides are best for snorkeling. Conversely a low tide at Neap Tide (half moon) is virtually not a low tide at all. So be sure to check a decent tide chart for exact tides. The "average tide charts" that you get from local stores and on your weather forecasts are normally very vague, just giving you the time for high and low, without saying what height the high will be and what height the low will be.
We use Tide-Forecast.com for our tide forecasts, and as you can see on this site, they include the actual specific maximum high height of the tide and minimum low's height. Ideally, for advanced snorkeling, you want a low tide that is LOWER than a 0,40m anything higher than that, and you won't have enough exposed shallow pools to explore (water will still be rushing in) For our beginners snorkel trips, we use much more protected pools, so you can get away with a low that is anything lower than 0,80m or so.
Another tidal consideration is incoming or outgoing tides. We find outgoing tides (about an hour before a max low) is best as all the silt and sediment is being carried out to sea, once the tide turns and starts coming in, even though the water may still be shallow and protected, the incoming tide brings new sediments and dirt into the pools.
Interestingly, tides and tide forecasts are NOT an exact science, many factors influence the actual tide height you experience from day to day. This is why we recommend a "real time" tide forecast such as www.tide-forecast.com compared to the rough guide you get on most weather forecasts or fishing brochures.
So the tides are probably the biggest factor, but wind also plays a major role in how clean the water and visibility is. Certainly around KwaZulu-Natal, a few days of strong North East winds makes the water dirty and greenish. This is from all the plankton rich warm waters from north as well as sediments from the Tugela River (around the Dolphin Coast) that get carried inshore during strong North East winds. (And it brings Bluebottles too)
Conversely, the South West Winds bring cooler but cleaner waters. The few days following a good South West wind are best and the days following a strong North East Wind are worst. But there are also many other factors that finally decide on water clarity and visibility, but this is a good rule of thumb. To find a good wind forecast, visit Windguru (any wind less than 10Km/h is probably negligible ....it's the blustery winds that make a difference)
The swell, in our opinion, after the tides, is the biggest factor to consider.
A big ocean swell can make a low tide seem like a high tide. We use Magicseaweed for our swell and surf forecasts. What you're looking for (ideally) is a swell that is less than 4ft or so, that's average. Even better 1-2 foot and you're almost certain to have an epic snorkel. Anything over 5ft and we can almost guarantee the conditions will be a bit rough. Flat seas also allow water to be calmer, so sediments sink to the bottom and not much is being stirred up.
Big swells can come out of nowhere and can often be caused by storms thousands and thousands of kilometers away, so one day the sea could be flat, the weather perfect and for no visible reason, change overnight to big rough seas.
Temperature, time of year
Strangely, the water temperature and time of year plays a very small role in what you will see. Winter, we generally have colder water (19 Degrees or so) But a much better chance of clean water and much better visibility (due to the prevailing southerly winds this time of year) BUT in summer, the water is warmer and tends to have a greenish tinge to it (due to the prevailing northerly winds)
Time of day
The earlier in the morning the better, we find that after 10h00 or so, many creatures such as Nudibranchs and Octopus become very difficult to find (we suspect the simply go to rest like most land animals do)
People also cause a lot of disturbance, scare things and generally have a negative impact on the variety you could see while snorkeling, early mornings means less people on the beach, which means less disturbance, which means more fun.
So, your perfect conditions will be a gentle westerly wind, a swell of 1-3ft and a low tide that is around the 0,20m mark. Having said all that, there are days where for no particular reason, the snorkeling is amazing, the water is warm, clean and everything is very active, other days, not so much. Think of it a bit like visiting a game reserve, some days you go out and see the Big 5 in a morning, other days you can drive around for hours and not see an Impala. The sea is very much like that, unpredictable. But we try anyway:)