Hi Joe, thanks for sharing some samples.
My first advice to you would be to standardize image acquisition. Some quick adjustments to your photography takes a lot less effort than accounting for uneven illumination etc. in post-processing.
Before we get into this, what do you define as fouling? The barnacles only, or the brown deposition as well? In the screenshot below, which area do you consider not fouled?
In any case, I think the trainable weka plugin will be the best tool for segmentation of these images. You can give it a shot on a cropped version of your image, make sure you use the same area (either the total plate or a fixed area) when you crop each image. You can crop from hinge to hinge like below, assuming all the plates have the same dimensions.
If you need more specific advice let me know!
A quick test using trainable weka segmentation, default settings and just two classes gives: