Cramond Island is an "tidal" island in the Firth of Forth. At high tide, it's an island. At low tide, there's a path you can walk to get to and from this so-called island.
You just want to make sure you come back to the mainland before the tide comes in.
Running just to the east of the path (the right in this picture) is a row of concrete pillars. These were built during WWII as a submarine trap, to prevent subs getting upriver on the south side of the island. (I'm guessing there must have been something on the north side of the island too, but I don't know what)