in the cave update, they emphasized underwater (or waterlogged) caves. glow squids will likely spawn in there, to add extra lighting and visibility to the area. It is though the squids will likely not really give off light, but rather glow like endermen or spiders' eyes, they would likely drop glowing ink that could be crafted into underwater torches or lanterns. The glow squid is going to spawn in those big lakes and little crevices, likely along with some sort of algae or moss. Thank u for coming to my ted talk