As an Aircrew rescue swimmer, we trained, sitting in the cargo door for a)called "10 & 10" which is 10' at 10 knots. B) "15 & 0" which is 15' at 0 knots speeds respectively. However, the 1st number was ALWAYS a reference point to the Top of the waves and the speed was also +/- 5 - 11 knots once you take into consideration wind direction & wind speed.  So if you have mild seas of say, 10' then your "10 & 10" could be as far as a 20'-ish drop. Is your in heavy seas, well. You get the idea now, right?