Re: oak vs. mahogany rubrail
No, you are quite right. The tannins in white oak turn it dark when exposed to water. However, it does steam beautifully, and can be sealed up pretty well. I usually cotton swab the screw pilot holes before install, and that helps... along with lots of varnish. My swimplatform edge is white oak, and has been fairly easy to maintain given its location. Mahogany works very well, and does not stain as easily, altho part of that is that mahogany is darker to begin with.
Wooden boats are like shingles, recurring, and often painful.