Hi DW,
Yes, the techy stuff can be a bit much for us creative types. I don't understand it most of the time either, but those who helped you today did well in explaining where to find things.
There is another site which leans more to the artistic side of things and someone there might recognize the filter or plugin right away. It's url is https://pixls.us/
I thought I recognized the filter but wanted to check it out first, so I wasn't wasting your time.
I downloaded a Public Domain image from Rawpixel that had windows in a boat, and some water that wasn't extremely calm.
Here are the filters I used (as suggested by the others). I'll tell you what I used for each one.
The original image
Filters > Artistic > Softglow (Legacy - means it an old filter)
Filters > Artistic > Oilify
Filters > G'MIC > I used the search box and put in Mean Curvature and it was there under the Heading 'Repair'
Filters > Blur > Mean Curvature Blur at the setting of 10 for Iteration as denzjos showed in his photo.
Filters > Artistic > Clothify (it's in the bottom half of the list)
Filters > Artistic > GIMPressionist (just below Clothify)
This is the one I thought it was. It is called Dream Smoothing.
It has more clarity than it used to even a couple of years ago. I use it all the time. I like the new clarity but it would have had the more blurred look of your image a few years ago.
For this image I used the Default setting, but you could play around with it to get it more blurry like your image is.
It's a G'MIC Filter
I don't know which version of GIMP 2.10 you're using. I'm using GIMP 2.10.34
The menu used to be:
Filters > G'MIC (In the bottom section of the Filters menu), but in my current version the menu is:
Filters > G'MIC - Qt...
Then G'MIC opens up in it's own window.
Artistic > Dream Smoothing
I hope one of our answers gives you what you're looking for.
AngelH.