They're the same but one is water; the other is land.
they're both a strip of land (isthmus) or water (strait) that lie between two pieces of water (isthmus) or land (strait) and connect to other land (isthmus) or water (strait)
In a way they are bridges: and isthums lets you travel by land from one piece of land to another, and it is this bridge, and a strait lets you travel by boat from one piece of water to another, and it is the canal between.