The semi-enclosed body of water can probably be called a reef lagoon.
A coral reef lagoon is that body of water that lies within an atoll (annular) reef or within a barrier reef. It is generally assumed that an appreciable depth of water is maintained within the lagoon, say 5-50 ...
The workflow of fusing RGB true color image and panchromatic grayscale image can help understand this question. After converting RGB color space to the IHS space, the intensity component is replaced with the (high-resolutional) panchromatic image, and then the new intensity, hue, saturation combination as IHS are converted back to the original RGB space.