It’s highly unlikely that anyone will serve you fermented shark. Although you might be able to find it if you’re curious and want a taste.
For Icelandic people, fermented shark is an ancient food that they have a connection with. Some associate it with their childhoods or have a fondness for it. But they understand that it is the ultimate acquired taste.
Fermented shark is a lot like stinky tofu or durian fruit in other parts of the world. So, just politely decline if you’re offered some but don’t want it.