There are 6 John Watersons baptised between 1814 + 1816 but in all the near 2000 Watterson children baptised prior to c.1850 there were no Fredericks!

Where did you get the marriage date etc - How do you know John Watterson was Manx (I agree it is very very likely) but knowing the source can help a lot eg if info came from Ancestry with 'about' tags then best treated as fiction.