Makeshift opener Watson finished on 62 not out after the teams managed to fitin 30 overs, with rain having delayed the start of play at Edgbaston.
Watson replaced the out-of-touch Phillip Hughes at the top of the order.
“He played very well,” Swann said.
“There was a lot of pressure on him coming in to replace Hughes and I thought he played very well.
“He made us bowl at his stumps and he scored. So we’ll have to come up with a way of bowling at him tomorrow.”
England’s only success of a run-heavy session came in Swann’s first over when Simon Katich paid the price for over-adventure as the ball snuck past an attempted pull to seal a straightforward leg before dismissal.
Australia closed on 126 for one, and Swann is hoping for an improved performance today.
“It’s a shame we didn’t bowl as well as we hoped,” added the spinner. “Weknew the Aussies would come out hard and did that.
“The delay was frustrating but the groundsmen have done a stunning job to get any play at all, to be honest.
“The pitch was fairly slow and flat. Conditions weren’t impossible. The ball got damp a few times but nothing out of the ordinary.”
Australia captain Ricky Ponting won the toss and opted to bat.
Australia initially made one change, as anticipated, with Watson in for Hughes at the top of the order.
But in a dramatic twist, they were forced into another after the toss took place when wicketkeeper Brad Haddin incurred a suspected broken finger in the warm-ups.
That handed an international debut to South Australia’s Graham Manou.
The quality of the surface was put into context early as Watson and Katich played their shots. Watson did enjoy one moment of good fortune when he jabbed down on a Flintoff delivery in the 10th over and sent the ball bouncing over leg-stump.
Eventually spin delivered the breakthrough as Swann struck from the city end with Katich four runs shy of 50.
Australia reached 126 for one by the close with Watson unbeaten on 62.