Possible Clues:

• Having been sworn in
• How a witness may appear to Noah in new guise
• 'You'd have sworn you were there (2,4)'
• You'd swear if you're there
• Sworn to speak the truth
• Where you'd swear you were not lying
• You'd swear it was the God's truth thus
• The truth, the whole truth and nothing but the truth said this way to Noah
• Sworn in
• Speaking truthfully, we assume, for boat-builder, temperature dropping