Source: Mats Bengtsson
Polishing a propeller on a ship helps to decrease the propeller surface roughness thus reducing the drag coefficient resulting in substantial fuel savings (upto 4%).
The cost of this service can range anything from $4k to $8k. Considering a tanker can consume as much as $5k per hour in fuel that's a huge saving.
Although the video description says it can pay for itself in a year I read that this service should be performed every 6 months and it may pay for itself in a single day of continuous operation. (article: https://glomeep.imo.org/technology/propeller-polishing/