Squid... available from any tackle shop, or a mackerel/herring cut into two chunks... oily fish travel's further.
If there's food, they will be too busy eating to damage the nets.
An alternative is to find a rocky shoreline and get out turning over rocks during a low spring tide. Just be very aware of the speed of the returning water, and the MSL for each type of crab.
Edible crabs (large brown) burry themselves in sand pools under rocks, so always have a good look and return the rock as you found it, putting the wet side back down.
Oh, and velvets make great eating if decent enough size, but they dont call them purple ninja's for nowt
From a pier with a crab trap, prepere for the odd 'monster' and you can very cheapley buy drop nets that are just a large metal hoop with a catch net underneath, you simply bait the middle, tying on the foodstuff, lower onto seabed, leave half-hour, then lift fairly quickly to entrap whatevers inside and eating. helps if you can look over and see whats going on as they do like to rob your bait given half the chance!
you can make your own with an old bicycle wheel, an onion sack (plastic) and some rope.
Bind sack with nylon cord around wheel, make long rope with 3 small end sections tied on, tie end sections one third around other side of wheel so it lifts uniform (like hanging basket)
A box of frozen squid will give you 10 or so good size baits for £2... and if you want to increase the scent trail, defrost them 48h hours before use in a sealed tub in the fridge, pour the liquid out once deforsted (overnight job) and then pour in a table spoon of common vegatable oil and mix through squid to carry the scent as far as possible. Slightly ripe (pink skinned) oily squid with have the seabed crawling your way.
Killer crab bait!
hth, TBL.