Here's something I found about mole crabs:
"they only spend part of the year at the surfs edge, then when the water temps fall during the Winter, they go to deeper water, and spend a large amount of time free swimming, according to one article. Also, when they spawn and the eggs hatch, they themselves are part of the plankton-culture. When they leave the larval stage they return to the surf's edge as "recruits" and intermingle with the adults. Also they are pushed down the beach by the currents and the wave, which means you may find large collections of them in one place and few or none in another spot. A couple of articles say the reason the live at the surfs edge is for protection from predators while they spawn. "
I am looking for more information, but apparently, from bits and pieces that I am finding, they do not require any land to crawl out on or direct access to air.