The moment that you captured is great with the fish in the beak. That is always a plus in avian photography.
To pull off a nice image, the moment has to be there, but the technical aspects also have to be there.
This image has a number of technical issues:
Exposure: The whites are so overexposed that there is no detail at all. Especially noticeable in the back and on the head. I don’t know if you shot this in raw and if there is detail in the raw file, but as presented, the detail isn’t there.
Focus: The image just isn’t sharp. I think there are 2 things going on. I don’t think the focus point was on the bird, and I think there is camera movement as well. The camera movement is pretty clear looking at the reeds in the background. They all have a blurred look. At 600mm on you camera, shooting at 1/320 is pretty tricky, even with any image stabilization. You could have easily taken your ISO from 100 up to 400 and your shutter speed would have been 1/1250 which would have helped.
I agree 100% with Keith’s points on this image. There is not much to add as I think Keith made great points on the technical issues. Shooting birds at ISO 100 is taking a big chance particularly with a 600mm lens. Just not enough shutter speed to get you a sharp image most of the time. Also, shooting from a kayak increases the challenge significantly of getting sharp images particularly with a long lens.
Thank you @terryb , @Keith_Bauer , @David_Haynes for all of your helpful comments. With the available light I easily could have increased the shutter speed with no ill effects.