Where Malia goes, the Secret Service goes. Fact of life. At least it was in Chicago, relatively close to DC (as opposed to another high $$$$ vacation).
They hired the locals because they wanted people who were out on the lake on a daily basis anyway. That way they would not attract attention.
I remember I was up in Kennybunkport, ME on my father in laws boat about 20 years back. A big Scarab boat came up the river followed by a 28’ inflatable with US Secret Service stenciled on its bow. The SS boat had three ex-military looking guys on it. It also had two 100HP outboards on the back. The agent on the bow was in a wetsuit holding a machine gun. When they came back down the river we noticed the George Bush Sr. was driving the first boat. He had a bunch of guys with him. It turned out the were the Oak Ridge boys.
We spent the night on the docks there right next to GW and the SS boats. The next morning GW and the SS boys came down to the dock said hello to everyone and then jumped in his boat and went back out on the water. We were told the SS boat could exceed 60mph if required.