Change the water often on hot days and keep the bucket in the shade if you can as warm water will cause the crabs some distress. You will have to learn how to pick up a crab ! They are fiesty creatures and the bigger ones can give quite a nip. Place your index finger on the back of the crab, you may have to distract it with your other hand while you do this; you can then gently press the crab down on its belly with that finger to stop it moving off, then place your thumb and middle finger one on each side of the shell. You can then pick it up whilst it scrabbles about clutching at thin air with its claws. Remember that crabs walk sideways.
There are several different species of crab that you may catch, shown here, the most common is the Shore crab which is green or brown and usually quite small. The Spider crab is a large spiky backed crab with long thin legs, these crabs are right girly things and don't try to nip you like the other types do and usually don't even bother to try to get away. Another type is the Velvet Swimming crab, this type has flattened rear legs to help it swim instead of crawl, then there is the Edible crab and these are sometimes, if you are in the right place, big enough to take and eat. The legal size for taking crabs varies according to the area you are in, usually about 10cm (4") across the shell.
If you are using a line with hooks it is always possible that you will catch a fish, usually a rockling, blenny, pout or similar but occasionally a biggy is caught, A lad had a bass of a couple of kilos in West Bay Harbour last year and I have seen a dogfish and a conger eel caught in Watchet Harbour on a crab line. Another lad caught a dead body on his crab line at West Bay in 2008 but that's another story.