Floating Islands are large magical constructs. Enchanted by the element of Water, they can act like ships, ferrying troops across the sees.
Please note that Floating Island has no listed damage on main description page. This means that while there is ranged attack listed, it's most likely disabled.
Armies containing Floating Island can not attack even if other units are capable of moving on water. I do not think there are any circumstances under which you could get into combat screen with Floating Island.
Summon - Unit was summoned and belongs to another world.