A few years ago, my wife and I went to Five Islands Provincial Park and participated in the Not Since Moses 5º run.
It is a fun event held on the floor of the Bay of Fundy. The tide goes out far enough to allow 6-700 people to have a 5 and 10 km run, and then the tide
comes back in and all signs of the race are removed.
It was so much fun that the next year, we took the kids and we did the race as a family.