Two bodies of water were merging in the middle of The Gulf of Alaska and there was a foam developing only at their junction. This happens when glaciers of fresh water start melting and flow to join the ocean water which is more salty. Because of the difference in the salinity and densities of these two meeting ocean water bodies, a surface tension is developed between them that acts like a thin wall and does not allow them to mix.
"He has let loosed the two seas (the salt water and the sweet) meeting together. Between them is a barrier which they cannot pass." [Quran 55:19-20]