From Ithaca, about 3-1/2 to 4 hrs.
Pick up I-81 North at Cortland (from Route 13) or Homer (from Route 281), passing through Syracuse and Watertown (where exit 45 is a good rest stop (gas, food shopping, zillions of fast food places), and on to the Thousand Islands Bridge ($2 toll). After passing through customs and immigration (if you have a pet it will need current rabies vaccination papers - a tag is not enough), continue across the second span of the bridge. Take Canada 401 West (toward Kingston) about 8 miles to the second Gananoque exit (sign also states to Crosby), which is Route 32 North. About 12 miles later this route ends as it T's into Route 15. Turn right (north) and immediately pull into the gas station across the road with a couple of phone booths. This is where you should plan to place a phone call to Exe Island to arrange for someone to meet you. A quarter (US or Canadian) is all it takes - a local call to our Kingston-based cell phone, 545-5108. [If you wait to call till you reach Portland, be aware it may cost you at least $3.75 from there, and you'd better have a LOT of Canadian change (the phone booths don't take dollar coins (loonies) or $2 coins (toonies).] Proceed north on Route 15 for 16 miles (passing at 4-mile intervals Morton, Elgin, and Crosby) till you reach the sign for the village of Portland. Look for a gas station on your left, and turn left after the station, and left again at the first opportunity. That will T into Water Street, where you make another left onto what quickly becomes the driveway of Len's Cove Marina. Drive to the Marina Shop area either to meet someone from Exe Island who will tell you where to park, or to arrange there for a water taxi to take you to the island (about $20 Canadian plus tip). They'll also tell you where to park.
From NYC area, figure on 7 to 8 hrs.
Best bet, unless stopping in Ithaca, is to take the NYS Thruway north (crossing the Hudson on the Tappan Zee Bridge) and west, where you can pick up I-481 North just before Syracuse, which will avoid the city and hook up with I-81 North. At this point follow the directions above for driving from Ithaca. Another alternative is to take the Lincoln Tunnel and pick up I-95 South to join I-280 West and then I-80 West through New Jersey, picking up I-380 North to Scranton, where you'll join I-81 North through Binghamton and Syracuse. At that point follow the driving directions from Ithaca, above. If you do stop in Ithaca, our choice of a route there is to use the George Washington Bridge, then the Palisades Parkway, picking up Route 6 (exit 18) to Harriman, where you take Route 17 West all the way to Binghamton, then I-81 North to Exit 8 (Whitney Point) and then Route 79 West to Ithaca.
From Ottawa Airport, about 1-1/4 hrs.
Exit the airport and turn left (west) on Hunt Club Road to a junction with Route 16 South (left lane). Take the exit (right) to North Gower (pronounced "Gore"), taking County Road 6 (later suddenly becomes County Road 4) to Smiths Falls. There you'll take Route 15 South to Portland. Turn right into the village and keep going down (right) toward the water. The street that hugs the shore is Water Street, which when you turn left onto it eventually becomes the Len's Cove Marina driveway. See the last part of the directions for driving from Ithaca for the drill from there.
You can download a copy of these directions for use when travelling.