The earliest reference to a tidal spring or well seems to be that by Pliny the Elder (Naturalis Historia, II, 100; c.77 CE)

At Gades, which is very near the temple of Hercules, there is a spring enclosed like a well, which sometimes rises and falls with the ocean, and, at other times, in both respects contrary to it. In the same place there is another well, which always agrees with the ocean. On the shores of the Bætis, there is a town where the wells become lower when the tide rises, and fill again when it ebbs; while at other times they remain stationary. The same thing occurs in one well in the town of Hispalis, while there is nothing peculiar in the other wells.*