Hey Lori...
I've had a place in Tobermory and I've been diving there for almost 30 years. Feel free to PM me with any specific questions.
Don't let the time of year scare you off... while it can be a little lumpy on the lake in the fall, the water is downright balmy... OK, maybe balmy is too strong a word, but even wetsuit types can be comfy in the fall. There is a thermocline at about 60-70 feet, below which the temperature stays about 38-41 F year round, but hey, how long are you going to stay there anyway? At least your stops will be in warm water.
Both shops in town, G&S Watersports and Divers Den are owned by good friends of mine and both are great operations.
Socially, the town does a little sleepy at night, so start early. The Crow's Nest and Captain Tripps both offer pub atmosphere, possibly live music and decent food. There are several decent motels in town as well as camping. Nights can be cool though in the fall, so pack warm clothes.
I use Messenger as well, so if you want to chat, PM me and I'll send you my "handle"
See you!
S