@dgar This prompted me to look up the etymology of island and it turns out to be Germanic through Proto-Indo-European, with 'is-' meaning 'thing in the water', with '-land', meaning 'land', suffixed apparently to distinguish it from other words that sounded like 'is-', such as 'water'. So it's 'land in the water'.

Also that it has nothing to do (except for spelling) with 'isle', which is from French, from Latin, from unknown origin.