The Grand Slam winner received a 49th-minute red card for what referee Greg Garner ruled to be a dangerous tackle as Saracens full-back Alex Goode caught a high ball deep inside his own half.
With scrum-half Chris Cook being sin-binned shortly afterwards, Bath were temporarily reduced to 13 men, and Saracens required no second invitation to take advantage as they triumphed 30-10 in bonus point fashion, opening up an eight-point gap on second-placed Exeter.
Saracens' England international wing Chris Ashton marked his return from a 10-week ban by scoring two tries, while flanker Will Fraser and substitute back Mike Ellery also touched down, with fly-half Owen Farrell kicking two penalties and two conversions.
Bath could only muster a George Ford penalty and conversion of substitute Leroy Houston's try in reply, and their final four league games of a dismal season could now be played with Watson, who helped England to Six Nations glory in Paris just 13 days ago, on the sidelines.
Garner consulted television replays before brandishing a red card, but Watson was furious as he left the pitch, reacting in disbelief at the official's decision and appearing to have words with Ashton, who had pushed him following his challenge on Goode.
Ashton, who had been sidelined since Saracens' European Champions Cup pool victory over Ulster in January after being cited for making contact with Ulster player Luke Marshall's eye area during that game at Allianz Park, was quickly into the action.
He gathered Cook's loose defensive clearance, setting up an opportunity for Saracens that they accepted in style as Farrell and centre Marcelo Bosch linked impressively before Fraser touched down from a resulting ruck.
Farrell's touchline conversion made it 7-0, before he landed a 40-metre penalty shortly afterwards to confirm an outstanding spell of dominance that saw Bath - Watson apart - struggle to find the same wavelength.
England forwards coach Steve Borthwick looked on as Bath tried to find an attacking foothold, but they were forced into a 20th-minute change when wing Horacio Agulla went off injured.
Agulla's exit meant an early appearance for Wales fly-half Rhys Priestland, who went to full-back, with Watson going on the wing, but it made little difference to the overall scheme of things as Saracens continued to enjoy control.
Another Farrell penalty made it 13-0, then Saracens executed a brilliant try underpinned through superb movement and precision passing, highlighted by Goode's long ball that sent Ellery over unopposed.
Bath had no answer, and even though Ford booted a penalty with the final kick of a one-sided first period, there could be no masking the fact that the home side had been horribly outplayed as they trooped off 15 points adrift.
Saracens replaced England star Maro Itoje at half-time - Itoje walked back to the bench with an ice-pack on his left knee - and Alistair Hargreaves took over from him in the second-row alongside George Kruis.
Goode then made a spectacular break from deep inside his own half, with Saracens rapidly shipping possession wide via Farrell, Mako Vunipola and Schalk Brits before Fraser touched down one-handed, but he had a foot in touch and Bath escaped.
Bath were an improved side in the second-half, yet Watson's dismissal ended any hope of a revival, and Ashton's 55th-minute touchdown reaffirmed the away team's dominance.
Bath kept plugging away, arguably playing their best rugby during the final quarter, and Houston's touchdown rewarded that effort, yet Saracens were already home and dry, sending them into next weekend's European Champions Cup quarter-final against Northampton with sky-high confidence as Ashton's second touchdown secured a five-point maximum.