Confirmed - cause found, patch not written yet.

Swimming into a slope makes the grounded check reject the very floor you are standing on, detaching you; swim gravity then sinks you into the terrain.

Moving it out of OPEN so the list reflects reality: this one is pinned down to exact files and lines, so what is left is writing and testing the change, not working out what is wrong.

Fixed - commit 7794a3e2, in the next build.

This turned out to be the SAME defect as #85 (the Henesys slope), so both are fixed together.

When you walk or swim along a slope the game re-checks which piece of floor you are on each frame. That check could never recognise the floor you were already standing on, so it fell back on a narrow 3-pixel tolerance - and once the slope rose faster than that per frame it let go of the ground entirely. In the Aquarium, swim gravity then sank you into the terrain before shoving you back out.

It now keeps hold of the floor you are on while you are still within its span, which is what the original client does. Measured against the real map data, 4,029 pieces of floor across the game were steep enough to trigger this.