If you're an idiot like me, this is how I ended up figuring out where to go instead of trying to drop from the top down thinking it was on the side of the cliff, and taking an additional 15 minutes to figure out.

Right next to where you'll find the Martial God Haeam's Tomb knowledge, there is a small sandbar. Run up alongside the right and swim to the junction of the waterfall and rocks (sorry for the Remastered quality, probably should have lowered it for clarity). You'll be able to jump and pull yourself up to the the ledge where the knowledge is found.

Here's a gist of how to get to that ledge. https://youtu.be/fCmtAPMnNCc

https://ibb.co/7yF6MNY
https://ibb.co/7JvbHYL
https://ibb.co/4NvHj88

When you finished on this platform, why not doing this other knowledge nearby ?
https://bdocodex.com/jp/theme/12449/

Walk under the waterfall until you only need to do a little jump
https://imgur.com/a/SDYE8vs