Hawthorn entered Sunday night's match needing to win in the vicinity of 90-100 points in order to leapfrog Brisbane into third spot.
That would have meant the Hawks (15-6-2) would face the decimated Swans in Sydney in the first qualifying final instead of minor premiers Fremantle in Perth.
Things were looking good for Hawthorn when they skipped out to an 83-25 lead midway through the third quarter following Watson's moment of magic.
Aptly nicknamed The Wizard, Watson leapt onto the shoulders of Liam Baker and pushed off powerfully with his legs to take a spectacular mid-air mark before crashing to the turf.
In the immediate aftermath, Baker and Watson smiled at each other in disbelief as they soaked up the moment, with Watson giving his opponent a hug after nailing the shot.
But the wet and slippery conditions meant Hawthorn always faced an uphill battle to push the margin towards triple figures, with the Hawks forced to eventually settle for a 16.11 (107) to 6.9 (45) win in front of 43,066 fans at Optus Stadium.
Hawthorn still finished fourth, earning a finals double chance for the first time since 2018.
Watson, who was worn like a glove by Brandon Starcevich for most of the match, finished with three goals and a goal assist, but hurt his left hand after being bumped into the fence by Bo Allan on the stroke of three-quarter time.
Hawthorn played it safe by resting Watson for the entire final term, but coach Sam Mitchell played down any concerns.
"He'll be fine," Mitchell said.
"He's not injured. We just managed him out of the game. He'll play against Freo."
Mitchell hopes Watson wins the mark-of-the-year award.
"He's the easiest player to coach in a sense because no one taught him how to do that," Mitchell said.
"He said, 'I haven't taken one for a while, so I had to had to get up there'."
Hawks co-captain Jai Newcombe was cleared of concussion despite the back of his head slamming into the ground following a sling tackle by Harley Reid in the final quarter.
Reid will miss the start of next season if he is suspended over the incident, but playing in his favour is the fact Newcombe was able to play out the match.
Gunston's four first-half goals broke West Coast's spirits and was a perfect way to celebrate his milestone game.
Milan Murdock kicked three goals from 23 disposals for the Eagles, but there was little else to smile about for the hosts, who finish a disappointing campaign with a 4-19 record and an 11-game losing run.
"Tonight was pretty simple, we fundamentally didn't play a great game of footy," Eagles coach Andrew McQualter said.
"I think we didn't adjust to the conditions in the first half in particular. We just fumbled too much, didn't didn't handle their pressure.
"And really, we probably got taught a football lesson in a lot of ways. It was a slog.
"The conditions made it pretty pretty difficult out there, but overall, not a great day."
