With skipper Michael Clarke (45), Phil Hughes (44 not out) and Brad Haddin (38 not out) also enjoying valuable time in the middle, the tourists ran up 266 for four in reply to the home side's 320 before rain ended play for the day just before tea.
Watson, already pencilled in to open in the Ashes Series, made his runs off just 94 balls, with a boundaries all around the wicket, primarily off the front foot. Craig Meschede was Somerset's most successful bowler with two for 55, while there were some fiery deliveries from teenaged seamer Jamie Overton, who ultimately proved expensive with one for 81 from 13 overs.
The Australians began the day on two without loss and suffered an early blow when Ed Cowan was caught behind off Gemaal Hussain for three. It was 78 for two when Usman Khawaja was comfortably caught at second slip by James Hildreth in Meschede's second over.
Watson moved to his half-century with a single off Meschede in the 18th over with the total 94 for two. He had faced 56 balls and hit 11 fours, the best a sweetly-timed straight drive off the same bowler that grazed the stumps on its way to the boundary.
The 32-year-old Queenslander continued to punish anything overpitched and looked on course for a century when getting a leading edge to Overton and offering a simple catch to Peter Trego at cover. It was 147 for three and lunch was taken with four runs added, Clarke having moved to 16 and new batsman Hughes on four.
There was no sign that Clarke's innings was his first since the Third Test against India in March as the Aussie captain played fluently in the afternoon session.
He was five short of a fifty when surprisingly edging a drive off Meschede and being caught by wicketkeeper Alex Barrow, diving to his right.
Clarke had faced 95 balls and hit seven fours in an encouraging one hour and 50 minutes batting that showed no trace of his recent back problems. From 212 for four in the 45th over, Hughes and Haddin batted with few alarms against a mainly seam attack, supplemented by the left-arm spin of George Dockrell.
Haddin hit two straight sixes off the young Irishman, while Hughes collected 7 fours as they put together an unbroken stand of 54. The rain came in at around 3.30pm and persisted throughout and beyond at early tea, forcing the umpires to abandon any prospect of further play at 5.30pm.
- Sports & Recreation
- Phil Hughes
- Craig Meschede
- Brad Haddin
- Jamie Overton