Spotting

Could also be the mermaid's purse of a skate.

Here's the Mermaid's Purse link that has been recommended. http://staffweb.itsligo.ie/staff/dcotton... So interesting. Perhaps you can identify from the photos. A wonderful spotting.

Moved to the category, Fish. :-)

Definitely some sort of egg pouch.

I canted completely tell by the photo, but it appears to be a dogfish egg. Search up mermaid's purse

I think its a shark egg sack search it up.

Shark eggs look like this too right?

https://en.wikipedia.org/wiki/Egg_case_(...)
I agree with Heather Reddig.

Its the egg casing of a manta ray or dogfish. Look up "mermaid's purse"