if you are just interested in the flood areas why don’t you just work with thresholds and make a binary image?
I don’t think image texture will help much in this case as water is alreay quite easy to identify. You could however try to filter your image so that black water areas become more homogenous.
Also consider conversion to db as this inceases contrasts in dark areas and might help to find a more suitable threshold between land and water pixels.
Have a look at these topics, play around with the histograms (use only small areas) and try if you find a suitable threshold in the band maths:
- How to determine appropriate threshold for flood detection (Sentinel 1)
- How to get flood mapping with high accuracy?
- Flood Map using GRD
- Preprocessing and Thresholding for Extracting Water
Also a very good tutorial: http://www.un-spider.org/advisory-support/recommended-practices/recommended-practice-flood-mapping