For land animals, island have a high degree of endemism because they are often isolated for long periods of time. Fun fact: In island ecosystems, because there are so many ecological niches open to the animals that make it there, things that are small will start to get very large (giant rodents, kimodo dragons, etc.) and because of the pressures of sustaining yourself on an island, big things will get small (pygmy hippos, elephants). Also, things that would normally have been herbivorous will expand into different dietary niches - i.e. in the Miocene, one of the smaller Italian islands was home to a giant carnivorous hedgehog (well, hedgehog ancestor).
This would work the same for fish in any isolated body of water. There are lots of Red Sea endemics - I would guess that this is because the Red Sea area has a very complex geologic history, with the southern outlet being closed periodically throughout history. The last time we can be sure there was a landbridge at the southern end of the red sea was during the Miocene, though there are others that think it has been present since then, especially during the early Pleistocene.
This has been very ramble-ly. In summation - any area that can be isolated will have a high degree of endemism. The End.
