Emerging from the pavilion with St Helen's score at 89-3, Watson cleared the boundary ten times as he raced to an unbeaten 105 off 59 balls (striking two modest fours in between times) as St Helen's rattled up 247-6 (Lee Brown 53) in their All Wight League match.
Still reeling from the onslaught, Shanklin promptly lost Corey Sanderson and Mark Woodhouse for ducks to Oli Stubbings (2-8 off eight overs), but recovered for a while through Mark Ringer (33) and James Lucy (28).
St Helen's couldn't keep Watson out of the action as the all-rounder trapped Ringer lbw when he came on to bowl. Shanklin waved the white flag, reaching 104-8 by stumps, Harry Carkeet taking 3-21.