The line of concrete blocks linking Horse Sands Fort to Southsea is of similar vintage to the Forts (completed by 1909); its purpose was to ensure that shipping passed close to the Fort. The term Submarine Barrier probably means an ‘underwater barrier’ rather than a ‘barrier against submarines’ since the first submarines had only just appeared at this time (First British Submarine (Holland class) commissioned 1903).
The Submarine Barrier is clearly marked by yellow posts and there are a couple of passages through it - the Outer Passage (approximately 1 nautical mile offshore) marked on the South side by a 3-legged concrete structure (a ‘dolphin’) which is clearly visible from a long way off, and the Inner Passage just off the beach.
The blocks forming the Submarine Barrier are exposed at low tide. It is possible to sail over the barrier at high tide (but check your tide heights carefully first!). It is also possible to sail between the blocks at low tide but it is not advisable.