The WRC board fence around my yard is 40 years old. Many of the cedar posts and planks are original, I replace a few planks each year. The planks my son installed as a teen over 20 years ago are still sound apart from the bottom edges that contact wet soil. The spruce rails don't have anything like the endurance of the cedar: I sister them and let the original rot off, which is very bad practice and I wouldn't do that if the planks weren't cedar.

I buy more planks than I need and set the best ones aside for boats; I lose maybe 25% of those to knots but the price works out good. Downside is a bit more scarfing as they are only 6 ft long, but I've become very efficient at that!

From the look of the grain the mills typically slash-cut the sapwood for planks leaving the heart-wood for posts. The sapwood is lightest and OK for ultra-light builds but heartwood is stronger. The planks rip nicely into vertical grain strips.

However, if I built boats for a living I would use better stuff to reduce labor cost.