Be careful: Things like the florescent plankton you describe, as well as the luminescent algae you can also see twinkling along the seascape can poison sea life and make humans very sick if they come into contact with it, so swimming is not advised.
I wouldn't go swimming anywhere along the seaboard in Thailand without wearing rubber dive shoes. Jai and I have stepped on everything from razor sharp mollusk shells to giant box crabs along the shores of Bang Saray - and if it weren't for the dive shoes we would have been in the clinic getting stitched up.