Summary
Having a water body and replacing the water with lava by /fill can generate obsidian instead.

Steps to Reproduce:

1. If necessary, create a 3x3x3 hole in the ground.
2. Fill the hole with water (you may use /fill ~~~ ~2~2~2 water or similar).
3. Fill the hole with lava using /fill ~~~ ~2~2~2 lava or similar.

Observed Results
The water is turned into obsidian, save for the corner block that is the highest and on the southeast of the obsidian cube.
From MCPE-103821 and further testing, if you use /fill ~~~ ~2~2~2 lava 0 replace water, the resulting cuboid will have a checkerboard/alternating water-obsidian pattern.

Expected Results
All the water should be turned into lava.