Well........
my first bit of thought is....why did you try to root it in water? I am not trying to be obtuse, but, when did you ever know a plant that rooted itself in water?
This is the problem with this method...which for some reason people seem to believe will always work but actually almost never does. The stem will start to rot from the water before the plant develops sufficient roots. AND, the roots that it DOES develop in water are WATER roots, not SOIL roots. They are a different set of roots. Water roots are much more fragile than soil roots. Water roots have to be converted to soil roots once you plant the plant. This is just another misstep in the rooting process. It can be done, but it is a time consuming process shepherding a plant from water roots to soil roots.
You are much better off just STARTING OUT in soil.
Now you have a section of your stem that has rotted. Hopefully, (I can't tell from the photos) this does not include the node.
You need to take a sharp sterile (sterilize with heat, or alcohol) tool and cut the rotted part of the stem off at lest a 1/4 inch preferable a 1/2 inch up into firm healthy stem tissue. Dip that in a powdered fungicide like CAPTAN. Place the cutting into a substrate made of the following:
1 part coarse perlite
1 part good quality soilless potting mix
1 part fir bark (Orchid bark)
1 part horticultural charcoal
you can also mix in any of these:
1 part LECA (Aliflor, compressed clay pellets), 1 part hydrated coconut husk chips)
Water once, then water judiciously, maybe once a week.
This is how I myself root all of my cuttings that I propagate from my variegated Monsteras. I also propagate top cuttings of all my Philodendrons and vining Anthuriums the same way. It has never failed me. I have NEVER rooted anything in water, and I never will.
These are some of my plants that I have rooted in this manner
