Cumulonimbus are associated with heavy rain, snow, hail, lightning, and tornadoes. The anvil usually points in the direction the storm is moving. On hot and humid days, these storms occur over water as the radiant heat from the land absorbs moisture from nearby water and rises to produce thunderheads. These storms can also indicate a cold front and may be preceded by squall lines, a row of black storm clouds. Wind shifts unpredictably and accelerates dramatically.
Lightning can occur for miles in front of a storm and after the storm appears to have passed.
If you have a barometer on your boat check it every two to three hours. A rapid drop in pressure means a storm is approaching. Close all hatches and ports. Head for the nearest shore that is safe to approach and duck into the lee of land. Put the bow into the wind and take waves at about a 40-45 degree angle.