Is there a certain place that the crabs hang out? Having to find 18 shells is gonna take a while at this rate. I ahve the helmet that I purchased at the docks and have the spawn on often. Do I need to get all of my characters these helmets before I complete the shrine of anephas?
Is it better to search for them on land or in the seas?

You don't need 18 - just 12 or even less if you decide not to have your whole party make the trip to the underwater city.

The best way to find crabs is to make use of your scouting skill (do you have this?). I would just sail around on my ship near the shore and when my scout said there were crabs, we'd park the ship and jump overboard to fight them. Didn't take to long to get all we needed. Some people also recommend trying to pickpocket the crabs while engaged in a battle with them.

Wyv

Pushing to top for crab hunters....

i was bored sailing circles around skull island to crab hunt. i parked the boat in a place i'd remember, and went underwater. i swam around with spawn frequency on often, and i encountered a lot more crabs than sailing the seas. plus, with the sea sharks and eels to kill while down there, you get some decent exp pts and its something to do other than just sailing and waiting on your scout to say "a giant sea crab has been here". just keep your eye on your pc's health meters , its a dive-surface-dive thing, but really, it saves your eyes from staring at endless water.