Edited straight from the Wikipedia entry on hermit crabs
As the hermit crab grows in size, it must find a larger shell and abandon the previous one. Many species form housing chains to get a new shell. When an individual crab spots a new empty shell it leaves its own shell and tries the vacant shell for size. If the shell is too big, it goes back to its own shell and waits near the vacant shell. As new crabs arrive, they do the same forming a group of dozens of individuals, holding on to each other in a line from the largest to the smallest crab. As soon as a crab arrives that fits the vacant shell and claims it, leaving its old shell empty, then all the crabs in the queue swiftly exchange shells in sequence, each one moving up to the next size.
The answer to the title question…because they do like to be beside the seaside, oh they do like to be beside the sea…