I think it's very important to qualify that recommendation - with polarizers your footage will become uneven and change any time you change the direction of the drone, and therefore it's angle to the sun. For most people they are not a good choice as they require a pre-planned flight path and you cannot change direction, again unless you are OK with uneven footage, somewhat defeating the purpose. If you set it up properly, yes there will be a very specific flight path you can use to take the reflection off the water. They are definitely not something you want to use as a general filter or just leave on and fly around as normal.
My advice would be to buy regular NDs, 8/16/32 and add a PL for fun later if you still want one after you fully understand how they work and their limitations on drones.