No attempt to be made to recover Booster 7. SpaceX will intentionally scuttle the booster by allowing sea water to flood the tanks.
If this does not work SpaceX will try to open vents or use a vessel to roll the booster to sink. A backup plan exists to shoot the booster with a firearm, creating holes to allow seawater to ingress into the tanks.
I'm led to believe the firearm method of scuttling has been used before on Falcon 9's that could not be recovered in the past.