We use to swim rivers with small lunch boxes and dive for them. Flip the largest rocks you can and catch them by hand and throw them in the lunch bag.
They taste much better than crab or lobster IMO. Boil them in salted water until they turn red and scream. Normally it takes about 5 minutes. Most of the meat is in the tails, but you can also eat the claws on larger ones.
Best places for them that I've seen on the mainland are Jones lake and the lower Vedder River. You can keep 25 per person which is more than enough for a good side to a meal (they went well with pasta). You typically get as much meat as you would in a prawn tail.