The store I use regularly in Ottawa is Kanata Diving Supply. Here is a photo of the store, as you can see it is very much a diver's store. Like many of the local stores, KDS runs Wednesday night dives in season, which I assist with. Just turn up at the store at 1700 hours, Wednesdays.
 

Another store I use frequently is Sea & Sky on the Prescott Docks. No web page but below is a photo of Rob Christie outside the store. The store is now owned by the folks of Seeway Vision charters and Rob Christie works at Kanata Diving Supply. You can always get an air fill here on a Sunday. If you are really desperate for air in Prescott, you can always get a fill at the fire station. You may have to go next door to the police station to have the fire officer called but they've always been ready and friendly.

In Brockville, there are two places you can get air.  One is Brockville Dive Centre on the corner of King Street and St Paul Street  (across from the Brockville Art Centre). The other place is Brockville Fire and Safety on Hyslop (behind the Bi-Way store on Park and Central)  This store is not open at present during the week-end but they plan to open on Saturdays and Sundays.

Ron McDonald's boat Li'l Diver is another well known sight on the river. His vessel is for large groups. One feature it has is a rail along the starboard side, at the water line which makes very easy to move to the bow line for descent in a strong current. I'll have a photo soon.

Another local dive store which runs a dive boat is Burton's Dive Centre, also on the web.

Below is a picture of Spencer Shoniker's boat which is based in Kingston, as Suspense Charters. It's not very fast but is very comfortable and the Captain is one of the St. Lawrence River's characters. An unusual feature of this vessel is the air storage tanks - they are out of an old submarine and could refill an army of scuba divers. Get Spense to show them to you. His number is 613-634-0550. You could also contact him through The Kingston Dive Centre 613-634-8464.

I also like to use Limestone Diving  in Kingston. They have small fast boats which are a lot of fun. Kingston Dive Centre has a roster of dive boats run by their operators. You can always call them and a boat will be found.

For good technical diving folks in Kingston there is Northern Tech Diver . Trimix available.

Kingston is a nice town. If you want to stay downtown in very inexpensive but different accommodation, then contact the Marine Museum in Kingston and get a berth on the Coast Guard vessel, Alexander Henry, which is now used as a hotel.

On highway 2, in Mallorytown, West of Brockville, Dan Humble'sDive Tech store is a surprising find  off the beaten track. Trimix is available here as well.

Other charters and boats are listed at the end of this page.

On the other hand, it can be a lot of fun in a small party to rent a boat for a day, or weekend and sleep out on one of the islands. I've done this several times and there a a range of marinas to rent from.

There are also several clubs in the region which would be happy to hear from you.

    Ottawa Beavers   613-523-7666
    Ottawa Owls   613-745-6444
    Seaway Valley Divers
 

Charters and boats:

 Limestone Diving, Kingston  800-286-dive
      613-547-dive
 Abucs Scuba, Brockville 613-258-7684
 Ron McDonald, Cornwall - Brockville 613-933-1362
 Thousand Island Pleasure Diving, Wayne Greene, Ivy Lea 613-659-3943
 Depth Charge Diving 613-791-DIVE diver@comnet.ca
 The Upper St. Lawrence Scuba Charter Association
 DND Charters     dndcharter@interlinks.net
 Suspense Charters, Kingston  613-634-0550
 Under Where? 613-349-7343
 Sea N Sky Scuba, Prescott  613-925-0308
 Explorer Dive Charters, Richard Walsh 613-634-5018
 Ducks Dive, Picton, Lake Ontario 613-476-3764
 Bottom Time Dive Charters - Brockville 613-961-9945
      bottomtime@sympatico.ca
 The Seeway Vision 1-877-SEEWAY 1
        613-925-3625 Seeway Vision
 Northern Tech Diver, Kingston 613-634-8464
      1-888-895-4647
 Underwater Bums 1-514-946-4774
       1-866-728-2228
 Above & Below 1-613-342-2277

 Limestone Diving, Kingston  800-286-dive
      613-547-dive
 Rockport Dive Centre   613-659-3471
 Kingston Dive Centre    613-634-8464
 Sea N Sky Scuba, Prescott  613-925-0308
 Prescott Fire Station
 Northern Tech Diver, Kingston 613-634-8464 *TRIMIX*
       1-888-895-4647
 Dive Tech, Mallorytown 613-923-5101 *TRIMIX*

Dewar's Isle of Rest Motel, Prescott 613-925-3228
Caiger's Resort, Rockport 613-659-2266
French Creek Marina, Clayton, NY 315-686-3621
Boathouse Country Inn, Rockport 1-877-434-1212
Diver's Den Bed & Breakfast 613-342-6110