Significant wave height
On marine charts, wave height normally refers to significant wave height: the average height of the highest one third of waves during a sampling period. It describes the sea state perceived by an experienced observer rather than every individual wave.
Individual waves can be considerably higher. In a large wave population, a wave close to twice the significant height may occur statistically, but this is not a prediction of a particular wave.
Period changes the experience
Two sea states with the same height but different periods do not behave alike. A longer period carries more energy and can produce stronger breaking waves as it reaches shallow water.
Direction is equally practical. A bay sheltered from northerly swell may be exposed to south-westerly waves, while local bathymetry and coastline shape alter conditions close to shore.
A forecast, not a beach measurement
Marine models describe grid cells over the sea. They may not resolve a small bay, rocks, harbour structures or local currents in detail.
For swimming, a small boat or fishing, combine height, period, direction, wind and official restrictions. Visual confirmation at the location remains essential.
This guide is educational. For safety-critical decisions, use current observations and follow the competent official authorities.