Are there any serious pathogens you need to worry about from splashing in a decorative fountain in a park? cold water and in wisconsin.

Unlikely.: The two important characteristics of the water that make it unlikely to be a problem is that it is (1) cold, and that it is a (2) fountain, which should mean it is circulating, always moving water. The water that would be much more likely to contain dangerous bacteria and other organisms would be warm and stagnant (not running like a river).