I have a number of images in the style of "split-shot" or "over-under shot" where the camera lens is half-submerged in water. I am looking for a technique to make the underwater portions of the images actually appear to be underwater. Refraction is the most important, but a light turbidity filter to simulate ocean water would be good too. Anyway, I'm looking for suggestions. Thanks.

You may get an answer here but if you are looking for a faster answer try https://discuss.pixls.us/.
They have gimp peeps over there as well

Fair enough, thanks.