This will allow the player to submerge into an underwater area which has a sloped floor.
Create submergible water. Set the below-water sector's 'first wall' with Alt + F
(shown as blue in the image). Now slope the floor of the below-water sector, making absolutely sure that the lowest point of the slope is touching the first wall. Move the below-water S
sprite close to the first wall, and angle it away from the wall. Then press the O
key on it to place it directly onto the first wall. Apply the same procedure to the above-water S
so it will be in the exact same location as the below-water S
.
Use Ctrl + PGDN
on the below-water S
to make it touch the lowest point of the slope.