The OP is using 'reefs' to signal unknown dangers ahead. Reefs signify a much greater potential hazard if they are hidden.

Shoals are no less perilous than reefs being '... natural submerged ridges, banks, or bars that consist of, or are covered by, sand or other unconsolidated material, and rise from the bed of a body of water to near the surface. Often they refer to those submerged ridges, banks, or bars that rise near enough to the surface of a body of water as to constitute a danger to navigation.'

In fact, the phrase I know like this - to signal unknown dangers ahead - is 'hidden reefs and shoals'.

The OP is using 'reefs' to signal unknown dangers ahead. Reefs signify a much greater potential hazard if they are hidden.

Shoals are no less perilous than reefs being '... natural submerged ridges, banks, or bars that consist of, or are covered by, sand or other unconsolidated material, and rise from the bed of a body of water to near the surface. Often they refer to those submerged ridges, banks, or bars that rise near enough to the surface of a body of water as to constitute a danger to navigation.'

In fact, the phrase I know like this - to signal unknown dangers ahead - is 'hidden reefs and shoals'. And I have just discovered that this is the title of the 18th in Alan Lewrie's Naval Adventure series!