Isopods are similar in size as related amphipods. The groups, however, can be distinguished by the fact that amphipods are usually tall and narrow (except for the skeleton shrimps) but the isopods are usually low and wide.
Isopods can be found everywhere in the sea from the seashore to the deepest depths. The most commonly seen species in the seashore and in shallow waters are the granular marine isopod (Idotea granulosa), which can be seen here, and Jaera albifrons (it does not seem to have an English name). They can be found all around Iceland.
Some isopods have peculiar lifestyles. The fish louse (Aega spp.) are flattened but streamlined parasites on fishes. They attach themselves to passing by fishes and suck their blood and body fluids. Legends have spun around the fish louses. Among other things, it should be possible to wish for anything if the fish are turned in reverse under the tongue that it then bites.
Other species eat strange things. The gribble (Limnoria lignorum), for example, eats timber. Along with the tree worm, which is actually a bivalve, the gribble was pest in the past when boats and piers were mostly made from timber.