The Royal Navy has its own language and terminology.  But British naval ships are very much in the minority when we consider the other users of the rivers, seas and oceans.  In fact, including all sea-going vessels from yachts to ULCC's [Ultra Large Crude Carriers] one wouldn't be too surprised at a count in excess of ten million spread throughout the globe.  Given our current fleet, and being generous, that's roughly 0.0000044 of hulls belong to the RN if there are only 10 million.   Obviously therefore, there is a need for a common language to be used on the high seas by all 'who go down to the sea in small boats' and others. [Figures, compilation of Insurers Liabilities]

Internationally, the BEAUFORT SCALE is well understood. It is a measure of WIND and goes like this:-

0 = Calm 1 = Light Air 2 = Light Breeze 3 = Gentle Breeze 4 = Moderate Breeze 5 = Fresh Breeze
6 = Strong Breeze 7 = High Wind/Moderate Gale/Near Gale 8 = Fresh Gale 9 = Strong Gale 10 = Whole Gale/Storm 11 = Violent Storm

and 10 - Hurricane.

The wind of course drives the sea and creates an Internationally recognised  SEA STATE.   The Sea State is reckoned as follows and is stated as HEIGHT of  waves in feet:-

0 = Calm [glassy] = 0 1 = Calm [ripple] = 2 = Smooth [wavelets] = ⅓ - 1⅔ 3 = Slight = 1⅔ - 4 4 = Moderate = 4 - 8
5 = Rough = 8 - 13 6 = Very Rough = 13 - 20 7 = High = 20 - 30 8 = Very High = 30 - 45 9 = Phenomenal = over 45

The Sea State dictates the SWELL and that is defined also in feet.  The higher the swell number measured 0 to 9, the more severe the swell period or frequency of swell.  Thus, swells considered to be '2', '5' and '8' are measures of increasingly uncomfortable sea-rides and give some idea, though vague, about PEAKS and TROUGHS.  The complex resolution of size of ship is the 'n' factor; a quick and short  swell [frigate/destroyer] <rides better over a short swell, and the Carrier fares better over the longer swell>  :-

0 = No swell 1 = Short or Middle 2 = Long All three conditions are classed as 'weak' less than 2
3 = Short 4 = Middle 5 = Long All three are 'moderate' 6 to 12
6 = Short 7 = Middle 8 = Long All three are 'high' in excess of 12

9 = Confused - plain and simple.