I sail all the time, so I see them all the time, but I just don't find enough mushroom to make the stew. I have a stack of 50 cooked sea serpent waiting for mushroom, and several stacks of stew waiting to be eaten.

If you're out in storms and/or at night, they are actually quite common, but not as common as the Leviathan. Those things stack up like cord wood. I'm constantly having to avoid and/or hop on them and make them go away. I recently had 3 so close that I was able to jump between them when I was getting them to sink. Those really are navigation hazards!