You can get through all of Lake Champlain unimpeded, but I'd be careful in the wide parts as that's dominated by motorboats and it can get pretty choppy in high winds. I know almost nothing about the Canadian side except that there is a dam somewhere along the Richelieu. And I only know that because NY and VT officials were trying to get Quebec to open the dam up one year when a Lake Champlain spring flood lasted into July.