If you take William's place of marriage as a likely birthplace, the IGI has 8 entries for William Bowden in Somerset - 6 of which state 'St Johns', but in Frome. This may be more logical unless you know of a connection with the Isle of Man. However, I would like to second Brian's word of warning - all 8 of the IGI entries were LDS member submissions. More useful to you/relatively reliable are entries which have the words ' extracted marriage record' which should mean that the information has been taken directly from the parish records (though slips can happen here too).

I would be tempted to stick to known localities to start with - are William's children, grandchildren etc concentrated in Someset or the Isle of Man, for example, or somewhere completely different?