The Hudson bay is a massive water way, an inland sea in all but name.
The freezing temperatures here, only warming up a touch in late summer, make the wearing of a dry suit almost a necessity. Make no mistake that this is going to a challenging area to dive.
Your regulators also must be in optimal condition and specifically designed to cope with the freezing conditions encountered in the water. It would be wise to carry out any servicing prior to setting out diving in these waters.
One area for setting out from would likely be either the small city of Churchill which is located in the western fringes of Hudson Bay in Manitoba Province. This has no port for boats though that we are aware of.
Other areas include Moosonee and the nearby Moose Island. Both of them being in Ontario province to the south east. Both of which are likely to have harbours.
Chartering a decent-sized boat with an enclosed area is essential for any longer-ranged dive-trips you have planned. A Rib might do for a nearby location of a few miles or so but if conditions worsen while you're out diving you could face unneccessary hardship.
Areas of interest for diving in Hudson Bay include:
Akimiski Island - A bird sanctuary is here making it likely the waters will have abundant fish life. It is in Ontario province.
Hannah Bay Bird Santuary - An section of coastal land in the south-eastern region of Hudson Bay.
Moose River Bird Santuary - A bird sanctuary located at the mouth of the estuary into Hudson Bay from Moose Island.
Polar Bear Provincial Park - This will push you diving skills a bit, you've got the likelihood of polar bears in the water when you start your dive and after surfacing the water. Good dive comms essential here.
Sanikiluaq - An unknown island off the east of Hudson Bay.
Tidewater Provincial Park - Within a few miles of nearby Moosonee this estuary island could prove interesting and easy to access.
Ukkusiksalik Canadian National Park - This is a real frontier area. You're in the North-West Territory, at the fringes of where the Hudson Bay ends and the Arctic Circle bring.
Wapusk National Park -