Leaving the marina,
Go east to Clayton, New York. There is a US Customs phone reporting station by the public dock. Just look for the big red building (the Thousand Islands Inn) on the shoreline when you head in. The US Customs dock is just to the right of that.
Once you have cleared Customs, you should take a look around the town. It has been named one of the "coolest small towns in America." How cool is that? It is also home to the Antique Boat Museum, a must-see while you're there. And if you think your houseboat is big, be sure to visit George Boldt's houseboat. It's part of the museum.
To Boldt Castle via the Lake Fleet islands and Eastern Grindstone Island
Follow out along the Bateau channel as above. When you pass the Howe Island ferry and reach the intersection marker, turn left heading east past the red marker J16. Beaurivage is on your right. This is an ideal place to stop for the night if you're planning an early morning return to the marina. (Mermaid and Aubrey are also good.)
Follow the channel markers toward Gananoque, passing McDonald Island on your right. After green marker J8, turn right heading south. Stay in the channel with the red markers on your right, passing the house that's as big as the island it sits on.
Take in the beauty of Thwartway Island
After marker J17 and past Hay Island turn left heading southeast. Pass between Huckleberry Island and Little Huckleberry Island. Tuck in close to Little Huckleberry Island and continue southeast. The large island ahead of you is Thwartway (Leek) Island. On the southwest side of the island are mooring cans. This is a beautiful and quiet spot to stop for the night. No generators are allowed so you have an undisturbed stay. (Remember that your houseboat appliances are powered by propane so no generators are necessary.) There are no docks on the island. If you have a canoe or dingy, you can get ashore at the little landing in the bay. It's a great island to explore.
If you continue east you'll pick up the markers and the beginning of the Lake Fleet islands. Here you'll find two of the prettiest islands in the Thousand Islands National Park: Camelot Island (appropriately named) and Endymion Island (also appropriately named if you're up on your Greek mythology.)
Several spots to stop for the night
These are both great places to stop for the night. Camelot has six mooring cans, two on the northeast side of the island and four in the most westerly bay on the south side. The docks are on the southeast side of the island. The best way to approach the docks is to go around Niagara Island to get on the south side of Camelot. (You'll spot Niagara Island easily because it has a distinctive, large pink house/cottage perched on the eastern side of the island. It's a good landmark when you're returning from the east.)
To get to Endymion, after you come around Niagara Island, turn east. You'll see the National Park sign of the west end of the island. Endymion also has mooring cans (four in a southern bay) and docks at the east end of the island.
Heading east continuing on your way to Boldt Castle and Alexandria Bay, you can follow the channel markers for easy navigating. On your right is Wellesley Island on the US side.
Heading under the 1000 Islands bridge
You can either go north of Ash Island or south, between Ash and Wallace Islands. Both routes have channel markers. These routes converge again as you approach the Thousand Islands Bridge and two more islands in the Thousand Islands National Park: Georgia and Constance Islands. Both have docking. Be sure to pay close attention to your charts if you navigate around these islands.
Continuing east, you'll follow the channel, pass Club Island and then swing south, past Mary Island New York State Park. Keeping Deer Island on your left, you'll head into the main shipping lane. (Be sure to keep a watch out for freighters: they're big and they don't have breaks.) Rounding the corner, swinging back west, you'll get your first glimpse of Boldt Castle up ahead on Heart Island.
US Customs and Immigration is on Heart Island so you enter the US right at the castle. (If you go, be sure to take your kids into the "playhouse" built for the Boldt children. It even had its own bowling alley.)
After your visit to Boldt Castle, you may want to go to Alexandria Bay for gas. It can get busy so give yourself time if it's a busy time of the year.
If you're looking for a place to park for the night in the US (before clearing Customs on your return to Canada) you can back track after Mary Island and head into the Lake of the Isles. It's sheltered and provides a good place to anchor for the night. The water is also great for swimming.
To Boldt Castle via Gananoque Narrows and Canadian Middle Channel
Follow the beginning of the router via the Lake Fleet Islands but instead of turning south at the green marker J7, stay in the channel heading east. And be sure to stay in the channel. Even though it looks like wide open water to your south, it can get quite shallow in places.
Keep heading east. On your left you'll see Gordon island, a great fishing spot we're told. Continue east and stay in the lane through the Gananoque Narrows. You'll see an osprey nest on a beacon marker. Maybe you'll see the osprey, too. Have the kids keep an eye out for them.
When you get to the green H23, you're in the Canadian Middle Channel and now you'll be following the same route as someone coming through the Lake Fleet Islands. See above.