Definition
Many marinas retain water behind a sill, a wall whose crest height is charted relative to chart datum, or behind lock gates worked at set times. Access depends on tidal height: with a sill crest 2.0 metres above datum, a vessel drawing 1.5 metres and wanting 0.5 metres beneath the keel needs a height of tide of at least 4.0 metres. Tide boards, depth gauges and traffic lights at the entrance show the state of access.
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