Sorry if this is already been posted but the search was not turning it up.

Can you draw line of site to soaring creatures hiding behind trees or stones on the open sea, so outdoors?

Soaring figures only benefit from the terrain they're directly standing on if they landso, for example, you don't get Shadowcloak from a tree if you're Soaring above it.

You still trace LOS normally, though, so a figure Soaring behind a tree cannot normally attack through it, nor can it be attacked through it. Nor can you attack a landbound figure that is standing behind a Soaring figure, nor can you move "under" a Soaring figure. Kind of like how you can't trace line-of-sight "over" a figure in a pit. The degree to which the Descent rules simulate a third dimension is very slight.