Tempest island has missing textures and entire areas you can go under map.

Armatesz

Area in question is literally the area you walk into right before going to stormfist... there is a sort of waterfall and the lake it forms on the opposite side if you follow all the way till the waterfall you should see a missing water texture where you are not swimming at. If you follow up the stream and go near one waterfall that the water looks rather deep and if you walk into it you literally fall through it, I had some friends of mine that came with me because I was farming a set from there for a test build and I showed them one spot so they can report the area and they discovered the other one as well.
You could literally fix the issues with a rock and a sort of ground right below the water. Looks like an easy fix and could have been an oversight. Only reason I was there was painfully obvious... looking to learn all chest spawn points. I've been wanting to learn all the chest spawn spots like I did for vaults of madness.