Waves are measured from trough to crest. Seas are the combination of both locally generated wind waves and distantly generated swell waves. Specifically, seas may be derived using the following formula where S is the height of the swell waves and W is the height of the wind waves.
SEAS =

Seas and waves are expressed in the terms of the significant wave height, the mean or average height of the highest one third of all waves in a swell train or in a wave generating region. It approximates the value an experienced observer would report if visually estimating sea height. When expressed as a range (e.g. seas 3-5 ft), this indicates a degree of uncertainty in the forecast and/or expected changing conditions (not that all waves are between 3-5 ft). Generally, it is assumed that individual wave heights can be described using a Rayleigh distribution.

Example: Significant Wave Height = 10 ft
   1 in 10 waves will be larger than 11 ft
   1 in 100 waves will be larger than 16 ft
   1 in 1000 waves will larger than 19 ft
   There are occasional reports of "rogue" waves of an even greater ratio

Therefore, assuming a wave period of 8 seconds, for a significant wave height of 10 feet, a wave 19 feet or higher will occur every 8,000 seconds (2.2 hours).

The danger presented to a vessel is a function of wave steepness as well as wave height and is unique to each vessel. In general for small vessels, for a given wave height the danger increases as the wave period decreases. Below is a table under development within the National Weather Service to assist forecasters in identifying sea conditions which may be of danger to vessels with a closed cockpit configuration of ~100 feet or less. The table is intended to be instructional only and the danger presented by waves to your own vessel may be quite different.

Reference: National Weather Service Marine Forecasts FAQ