Ok, underwater images have a very particular way to be processed (no Hue-saturation!!!), if you're a diver you know that after 5 meters, no more red, after 10 meters no more yellow and all in between with red, and so on depending the sun light.
Having said that to get a proper white balance you can try Colors > Auto > White balance, some time GIMP does a good job with it... (but it add too much contrast, but it can be surprisingly good)
But to be sure, here is how I do it manually:
On the original image > Color Picker tool > take something that YOU KNOW is gray (or white, but white might be blown out...), not dark grey, but middle/light grey
Then New layer (transparent) > fill it with that color (which is blue-ish or green-ish depending where you did dive or the depth you dove) > put it in Mode "Divide" (play with transpency ~50%)
If you are not very satisfied, duplicate that layer (the blue-green-ish one) then > Color > Invert > put it in mode "Dodge" > play with opacity ~50%
(don't forget to untick the eye / visibility of that blue-green-ish layer below)
If you are not satisfied, yet... > play opacity with both colored layers ~30%
once you satified
Then right click on a layer > New from visible > play the light/shadow with the curves or the levels (and ONLY now you can use Hue saturation if you wish)
Having said that to get a proper white balance you can try Colors > Auto > White balance, some time GIMP does a good job with it... (but it add too much contrast, but it can be surprisingly good)
But to be sure, here is how I do it manually:
On the original image > Color Picker tool > take something that YOU KNOW is gray (or white, but white might be blown out...), not dark grey, but middle/light grey
Then New layer (transparent) > fill it with that color (which is blue-ish or green-ish depending where you did dive or the depth you dove) > put it in Mode "Divide" (play with transpency ~50%)
If you are not very satisfied, duplicate that layer (the blue-green-ish one) then > Color > Invert > put it in mode "Dodge" > play with opacity ~50%
(don't forget to untick the eye / visibility of that blue-green-ish layer below)
If you are not satisfied, yet... > play opacity with both colored layers ~30%
once you satified
Then right click on a layer > New from visible > play the light/shadow with the curves or the levels (and ONLY now you can use Hue saturation if you wish)