Coral Reef without improvement has movement cost of 4, so I guess its to make bit different compared to Reef - easier to swim trough.Yes, and with the coast, a total of 7. 7/4 = 1.75 movement cost for those with double movement through reefs, allowing even a 2 move unit to get through it since he'd have .25 remaining after moving onto it. Any remaining amount allows another move. This is so that anything that doesn't have the double move for this sort of tile is pretty much bound to get stuck on it otherwise.