Currently I only have one sail but I am planning on adding a jib and a spinnaker (possibly as upgrades the player can get throughout the game).
The prevailing swell of the waves is based on the current wind direction but that information is already conveyed by the flag. I'm assuming that's what you mean by "reading the water".
Since I'm not planning on making it super realistic, I was thinking of adding a jib and spinnaker as potential upgrades to increase the speed of the boat at certain angles. The extra sails would move on their own based on the angles and wind speed.
With regards to hiking and falling out of the boat, I'm leaning more towards a disembodied camera instead of a first-person character so probably not.
I'm not sure if I'll add reading the water as a thing but I'll keep it in mind!