In my experience in whitewater recoveris prussiks are used to attach carabiners or pulleys to the rope rather than as a one way sliding loop.
My personal sequence would be to try a direct pull, if unsuccessful anchor the rope and try a vector pull, if still unsuccessful use a prussik to add a pig rig.
There is an excellent analysis of the forces involved at http://chung.ayjscience.ca/phy4561/Physics%20JA/Force-River%20rescue.pdf
Gedi Convertible Helmet
URCHIN Portable Anchor
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