You MIGHT be able to use a graduated neutral density filter to tone down the brightness however its use in your situation would be problematic because:
1. Often shooting into the sun with a filter over your lens will increase the risk of additional flare. You are already experiencing flare in this image.
2. It might be difficult on a moving/rocking boat to place the cutoff of the GND exactly on the horizon.
Hi Wayne, if you can do some post processing, shoot a normal and an underexposed copy of the same scene with the sun toned down, then softly blend just the sun portion of the underexposed image back into the normal one. I'll take some practice and may not be perfect but you'll learn along the way.
I hope you don’t mind, I have tried to adjust this photo in an effort to produce what I think you might want.
I generally do not use anything but a UV filter on my cameras and do everything else in post processing. The reason I do not use filters on the camera lenses is because, I then have the option of trying out different digital filters without having to deal with removing the permanent effects of a physical filter, should I decide I do not like the effect. For example, a star filter tends to “fuzzy” everything, when you might want the subject sharp and still have a star from the light source.
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Adjusted exposure and colours to simulate a polarizing filter and the golden glow of near sunset—which I imagined you might have seen. Since you already have a nice lens flare, I thought that incorporating a star filter (Photoshop brush) over the sun might work well here. I used a very subtle filter @ about 70% opacity. You could play around and consider a more intense star filter for a different effect.
BTW, I think that lens flares are often very nice, especially on photos of the sea.
I use Lightroom for most PS adjustments and Photoshop for special effects. You can find free brush/star filters all over the net.