I don't recall the full episode but a few things stood out.

• it would be cold as hel, so properly strapping a lifevest around some flotsam seems iffy
• isn't saltwater more buoyant or is that old fishwife's tale? Also idk if that test body of water is salty.

Yep, saltwater has more buoyancy, and the higher the salinity, the more buoyant the water.

Only if they had the knowledge and abilities (and time to come up with a plan) of two people whose job it is to come up with exactly this kind of thing.

Sure, it was possible. That doesn't make it probable.