What's in the cave? Is there a skeleton in/on the cave or if X marks the spot then why is that the first? Is there a ship in the ocean. The ship is upright so it's not sunken. Is that last one the Tree of Life or what's with the root system? Ok, so here goes - At the bones in the cave, walk 23 paces toward the shore directly into the sun at sunset and it's at the tree? OR read from right to left, which makes more sense - stand at the rooty tree facing the sun as it sets/rises depending on which side of the island the ocean is on, and walk 23 paces to a mound dirt or rocks which marks the spot.
Treemale is pretty easy to spot out there. The 8 under the x could easily be 18, or more likely, 8 with the x marking the spot. The three math lines are telling a substitution code that’s likely the next close.
The ship+arm and sunset... It’s also possible that shiparm would mean something different with a substitution code.
Me, I’d dig 8 paces west of the treemail and likely find the hidden immunity idol.