When I worked with the British Antarctic Survey in Cambridge years ago, they had some of those giant isopods in a tank. Seawater at about 2 degrees centigrade. They are really very slow moving and inoccuous, and we used to reach in and pick them up to show visitors. Think woodlouse, only aquatic, and about a foot long. Great things
For other weirdies, try googling bootlace ribbon worm, and looking at the wikipedia entry. A 180 foot worm! And it occurs on shorelines in the UK. We used to get little ones up on the Clyde coast in Scotland, and give one end to tourists and get them to walk away, whilst we paid out the worm like a rope. 10 feet long was quite normal, and that's before they start to stretch.