Dolomedes are large spiders, especially the females. This particular individual is almost certainly tenebrosus, the largest native spider in Minnesota, but there are about 100 species worldwide.
These spiders are most often found at the edge of water in forested country.
Fishing spiders received their common name from their habit of detecting prey--insects, tadpoles, even small fish--and then breaking the surface tension of the water to capture it. It is often found on or near piers, rocks bordering the water, etc.