After Artabazus had continued the siege by the space of three months, it happened that there was an unusual ebb of the tide, which lasted a long while. So when the barbarians saw that what had been sea was now no more than a swamp, they determined to push across it into Pallene, And now the troops had already made good two-fifths of their passage, and three-fifths still remained before they could reach Palline, when the tide came in with a very high flood, higher than had ever been seen before, as the inhabitants of those parts declare, though high floods are by no means uncommon. All who were not able to swim perished immediately; the rest were slain by the Potidaeans, who bore down upon them in their sailing vessels. The Potidaeans say that what caused this swell and flood, and so brought about the disaster of the Persians which ensued therefrom, was the profanation, by the very men now destroyed in the sea, of the temple and image of Neptune, situated in their suburb. And in this they seem to me to say well. Artabazus afterwards led away the remainder of his army, and joined Mardonius in Thessaly. Thus fared it with the Persians who escorted the king to the strait. -- Herodotus
Never a good idea before a swim or crossing a marsh.