Looks really good - it’s definitely possible to make this without plugins although it’s a bit more awkward to get things like wakes / reaction to objects.
No, it´s done by VRay-displacement with a noise-mix-map - based on the ocean-material by Pietro Canin (Vray-materials.de) - I modified the texture somewhat and added animation. Additionally a wave-spacewarp provides bigger waves.
I do the same thing - a high poly plane with a large noise / wave space warp to make the big rolling waves, vray displacement with a noise map for the crested parts, and a noise bump for the small detail.
To be honest I don’t bother with a lot of refraction in my sea scapes - a lot of the time they’re lit by the reflections as much as anything else - normally a minute a frame or less. Of course if you want to do a waterline that has nice sea through green water nearer the shore then you’ve got to start using fog which is a tiny bit more expensive. You can cheat for the peaks / colour changes using falloff maps or with colour changes on the maps driving the water displacement too.