(This issue was updated, see the comments.)

If a player with "slow falling effect", begin to walk from a "not waterlogged lower slab", into water that has the same altitude of the slab, the player will simply walk on the water just like on the ground.

The player should be sinking in this situation.

Here is how to trigger it:
1, find or make a pool.
2, place a lower slab in the top water block somewhere.
3, take out the water from the slab.
4, stand on that slab, give yourself slow falling effect.
5, walk onto the water around you (not run or jump).
Then you can walk or stand on the water, just like on the ground.