That may be a challenge, or it may work, depending on a) the quality of imagery in the basemap and b) how complicated the water boundary is (and if wetlands are also included).
Presuming your area is not small (more than a few square miles), rather than trying this as described, I'd suggest
A) try using NAIP color infrared imagery (see USDA NAIP image service in ArcGIS Online) presuming you are in the USA, or if outside the USA use a Sentinel-2 image (color IR band combination) for a pre-flood date (see https://livingatlas2.arcgis.com/eoexplorer/). Color IR provides an excellent delineation of "water/non water".
B) don't try to extract a shapefile or polygon - just do this work in the raster domain
C) you didn't mention what your "post-flood" imagery is - drone or possibly satellite? As noted, Sentinel 2 might give you a good post-flood view although cloud cover may be a problem for your specific dates, and also flood waters aren't as easily delineated due to the heavy sediment load...
If you get good results (and are allowed to share them), post an update here!
Cody B.
