First, you’ll need to head towards that high tower in Riplash Ruins (56, 88) and dive when you get to the small rock jutting out of the far left corner. You only need to dive a short distance to see a few strands of long green seaweed and then a couple of purple outgrowths along the narrow crack in the wall. This is the entrance to the cave. Swim through the crack, following the wall until you find an opening above you.
The egg is sitting in a pile of debris located behind the sparkling huge pearl at the back of the cave. If you’re having trouble locating the entrance, check out the video.
Congrats on your new murloc!