Symbols - What does heaven look like
An isthmus is a special sort of connecting portal between an island and the mainland of matter. It may be seen symbolically as a spit of land connecting the island surrounded by the ocean and sea to the mainland.
A causeway is in this context also an isthmus.
If you are the island, then the spit connecting you to the shore is keeping you anchored to the wider mass of the physical - in effect it is tying you to the physical. Unless this isthmus is washed away you cannot - symbolically speaking - become a boat!
But if the island is representing some deeply revered god then it becomes a symbolic bridge to that god - the means by which one passes from the physical world of the mainland to the deeply spiritual world of the god.