Public profile page: requesting user ID with dynamic url routing
.first() is going to be required... I updated the answer in the original question, but here it is for posterity, too:

{% set notWorkingUserId = craft.request.getSegment(2) %}
{% set notWorkingUser = craft.users.id(notWorkingUserId).first() %}


Then you can access the properties of that user because it's a proper UserModel now:

notWorkingUser.email