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Mass Ant Drowning #2

This is the strangest ant phenomenon I have ever witnessed. Walked along the shore at Sunrise Beach, Muzenberg at about 8 am this morning [nearly high tide]; for over a kilometre all along the wrack line were countless thousands of ant alates - both males and females. These were evenly spread for nearly a kilometre along the coast, ie the numbers were staggering. We collected samples from the wrack each a hundred or so metres from the next, for further examination. Back home we found that the ants were all the same species - possibly a Pheidole, possibly P capensis - I have sent samples to Peter Hawkes and will report later on the species. Interspersed were a few bees of unknown type, but otherwise there were no other insects in the wrack line. Ants were still being washed ashore as we walked, and back home we found that several - especially female alates - were still alive [and several hours later some still are]. This suggests that they had not been in the water for long, as ants' recovery time from immersion relates to how long they have been 'drowned'.
There were no strong winds and it last rained two days ago, although yesterday morning was overcast, cool and humid, the kind of day when alates might well fly.
Any suggestions around this mass drowning? One wonders whether they were not from a species which had infested a ship, perhaps? If so, the phenomenon suggests that this is one way in which alien ants could be spread around the world [especially as some female alates survived - but don't worry, they're all in ethanol now!]
The only other ants we've ever found on that beach before were occasional drowned male Dorylus.