The 30,000 Islands is a big area (as the name kind of implies). A paddle boat or rowboat is not going to cover more than a tiny fraction of them. However if that's OK for your uncle, then there are several places you can rent boats in the 30,000 Islands area.
Essentially you need to look for a marina on Georgian Bay rather than some other lake. Two places that I know have them are:
I would expect to find facilities at Midland, Port Severn and Parry Sound as well.
Honey Harbour has the advantage of being right in the middle of the islands. At Midland or Penetanguishene there is going to be substantial open water between you and the islands, which may not be a good idea for a beginner boater. If you stay at the Delawana Resort you can borrow canoes, kayaks and pedal boats for free, and rent fishing boats. However it's in one of the more crowded areas of the 30,000 Islands (in terms of boating traffic).
Unless your uncle has an extremely small car, it probably can carry either a canoe or a kayak. Canoes fit easily on compact car roofs, and the equipment to do so can be very cheaply rented from any good canoe rental place. If he has not done much boating you uncle might also consider renting a small boat with an outboard motor. They are rented frequently to fishermen, and are probably safer than a rowboat or paddleboat, as well as allowing you to see much more of the islands.
Finally there are good boat cruises round the 30,000 Islands out of Penetanguishene and Midland.