As a general rule xml is not case-sensitive for column-names, but you've used ModifierID instead of ModifierId so that would be the very first thing I would alter.
Also, use RequirementId instead of Requirementid.
I can't see within the code of the Cornwall_Traits_only_boat.xml where there are any syntax errors or missing requirement etc elements. Beyond the Case-sensitive possibility, what if anything pops up in database log ?