@airlinepilot ....this is not an answer to your quest, but I found your emergency water ditching planning/assessment very useful information. I work as an aviation meteorologist for an airline with international routes and I will now follow this up with our own internal emergency operational procedures manual. I hope your question never has to come across my (or any others) desk for assessment/clarification. Safe flying!!!!

Just an added note: This weblink gives a pretty comprehensive outline of wave modelling in general and some of the wave products from the ECMWF weather agency (modelling found within Windy)

https://confluence.ecmwf.int//display/OPTR/Our+training+resources?preview=/19661038/30246095/Jean_Bidlot_2014.mp4

I'd probably watch from 00:28min through to about 0:40min