https://www.ancestry.com/name-origin?surname=isham
https://en.m.wikipedia.org/wiki/Isham_(surname)
I found this interesting on the surname isham.
English: habitational name from a place in Northamptonshire named Isham, from the river name Ise (of Celtic origin) + Old English ham ‘homestead’ or hamm ‘promontory’ or ‘enclosure hemmed in by water’.
Source: Dictionary of American Family Names ©2013, Oxford University Press