Why a gap between islands can be too shallow for a ship

Which straits or sea areas look navigable on a common map but are deceptive?

Some gaps between island chains are very shallow; the example discussed is a Scottish passage that can be crossed on foot at low tide.

An apparently wide gap on an ordinary map can conceal water too shallow for a large ship. The example is a Scottish island passage that looks hundreds of yards wide, yet can be crossed on foot at low tide with little more than the feet submerged. The location is tentatively associated with islands around Skye after uncertainty over whether it is in the Hebrides. The distinction is between visible horizontal space and the depth required to pass through it.