Any good seascape painter must know the anatomy of a wave, why and how it forms, what shapes it and how it moves from swell to breaking over to foam to scud. The internet is the best place to find this or I highly recommend E. John Robinsons' books especially his "Marine Painting in Oil" where it is all laid out in diagram form. For beginning seascape painters this is the very best book out there, to cover all the aspects of painting. It holds up well though written some time ago, and works for acrylics or pastel as well.
Few of us would ever get this view in the barrel of the wave as it curls over. The website this comes from is one of my favorites, pointed out to me by a reader of this blog. Magic Seaweed gives me all the surf reports for the area I live in, and daily has photos and videos of the sea and waves all over the planet. If you go to their homepage and scroll down on the right you can find surf reports and information from all over the world and select your local area. If you are lucky enough to live near the water, you should be able to find something near you.
I check these reports for days I want to go paint, where the surf is, how big it is, and what I'm likely to find. It sometimes is not quite what the report says, but most days are close. I also have bookmarked a cam put up by the Bodega Marine Lab from Davis CA, This cam is 24/7 and is updated every 15 min. So I check to see what kind of day it is, if the clouds are good, or clear enough to see the sunset. Of course sometimes it changes very fast, and what was clear at 2pm is completely fogged in by 3pm, but it gives me a fighting chance. I live about 25 miles from the Sonoma Coast so I'm blessed with having my subject very close at hand whenever I want. I can't imagine how anyone would try to seriously paint the sea unless they had a way of observing it closely day after day. If you are landlocked and hunger for the sea, I hope someday you choose to own a painting of it, from me( check the top link of my blog for Currently Available) or other artists who really know and love it and have spent the time to faithfully capture its essence not from a photo, but by really being there.