Manual Binary Mask in ImageJ

Hi everyone,

I am a newbie to imageJ and image analysis in general.

I have a landscape photo that I want to analyse, and the first step is to create a binary mask, marking the non-water area as an area I’m not interested in, and the water as an area I am interested in. Ultimately I am trying to carry out auto-detection of boats in a large stack of images. I can see how to automatically create a binary but the result isn’t detailed enough, how do I do it by hand?

Thanks for your help!