I’ve never tried this. It certainly sounds interesting. I’ll give my thoughts.
It would be easiest and most realistic to let your actress actually get into the water. For safety’s sake it would be wise to have a SCUBA equipped diver nearby along with a lifeguard or two.
Unless of course, it’s too dangerous. Just how “tumultuous” is this ocean? If it’s too dangerous to put a woman in it, how are you planning to film it?
What is the woman going to be doing in the ocean? If she’s going to be struggling with the water ie. thrashing around, it’s going to be difficult to make it look realistic to comp a pool into ocean. I’m not sure how well a chromakey would work in a pool. Many pools have a blue color already in them which is nice, but I don’t see that (or your color) working very well, not because it wouldn’t key, but because the ocean isn’t as see-through as most pools are. Do you have a pool that has a similar level of visibility to your ocean? Perhaps you could film her in the ocean on a calmer day/calmer location with a green or blue screen behind her and rotoscope the front of her.
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