In Quentin Tarantino’s latest film Once Upon a Time in Hollywood, in a flashback, we see
Brad Pitt’s character on a boat with his wife, with a snorkel mask, some kind of long pipe or gun, and some sort of pressurized can.
From the context and dialogue it’s clear that these items are used to bring about a death in some fashion (for which that character is not caught), but the details are not revealed.
What are these items exactly, and how were they (most likely) used to accomplish this murder in the film’s universe?