The Chapel marks the spot where William Tell jumped off the boat of his captors as it was getting ravaged by a storm, as shown in one of the four frescos by the Swiss painter Ernst Stückelberg that are inside the Chapel.
I hope William also rescued the dog on the boat.
William's captors worked for Gessler, the Austrian overlord who had forced William Tell to hit an apple placed on the head of his own son with a single shot. After William's success, Gessler noticed a second arrow in William's hand. "You were supposed to hit the apple with a single arrow. Why did you need two arrows?", Gessler asked. "If I had killed my son, you would have been next", was William's honest reply.
Gessler got very upset, visibly riling up his dog.
Enraged by William's words, Gessler had him arrested and ordered his men to take him to his castle across the lake. As a massive storm broke out, the crew freaked out and unchained William so he could steer the boat to safety. That's how William managed to jump off the boat and, I hope, save the dog.
Note that the dog on the boat is different from the dog in the apple-shot scene.
Dachshund Daddy's travel note: the Tellskapelle, or Tell's Chapel, can be reached with a quick scamper from the Axenstrasse between Sisikon and Flüelen. You'll most likely use the Axenstrasse if you ever drive across Switzerland north to south. A much better way to visit the Chapel is to use the lakeside trail from the village of Sisikon. You can also take a ferry from Flüelen or Lucerne to the landing dock right by the Chapel...but check the weather forecast first and be sure no storms are expected.