Making an educated guess.......
Im guessing we were talking warm waters?
If so, then I think we're onto something - gases dissolve more easily in warmer water. A change in atmospheric pressure (from high/er to low/er) could theoretically result in the phenomenon you mention. Just as when you open a bottle of soda, the bubbles come out.
If so, I'm betting it was carbon dioxide (just like soda and champagne) - particularly as you didn't mention whether there was a smell present.
CO2 (or any of several other gases) is given off by volcanic action as well, so it may have been a crustal source of gas bubbling up through the ocean.
A last guess is that it may have been oxygen being given off from an unusually high concentration of algae.....
(I can't sign off without asking whether you and your bathing colleagues had previously eaten a few batches of spicey Costa Rican food?? There again, you didn't mention any smell did you....)
Thanks for bringing it to our attention.
stales
