Varenna group boat tour suggestion

We stayed in Varenna also, and we used the ferry system to get to all the other towns. We also rented a small boat by ourselves one day - the vendor gave us a cartoony map showing all the villas we could motor by. The boat was small, well kept, easy to operate. We brought along a lunch and had a great day. We also did the same thing in Cinque Terre, from Vernazza. (I saw your question regarding Lake Como.) If you're comfortable with renting a boat, it is really pretty easy. Also, in Vernazza, we wandered the docks looking for a small tour, as you are seeking. We did find a cool boat with room for about 8 persons, with a man who would take you out, dive himself, and come back with treasure from the sea to eat. He had a great dining area set up on board. We almost signed up, until we realized he spoke zero English and the other guests would be Italians. We felt we'd enjoy ourselves more on our own tour due to the language barrier. On that one, we were shown a "secret" waterfall, jumped overboard, and felt like millionaires on our own tiny boat, watching the stuffed ferries go by while we "charted our own course" (the boat owner helped us figure out the route). All prices were reasonable, and most companies have guided small tours also. Enjoy!
https://varennarentaboat.com/

Unless you absolutely want a private boat tour, get a full-lake day pass for the ferry system and ride it all the way north to the last stop at Colico. It's a very scenic ride, and Calico is a nice little town to walk around. If any towns look interesting along the way up, stop there on the way back to Varenna.

Another option would be to get a mid-lake ferry pass, as there are several beautiful towns and villas to visit in that area around Varenna as well.

Bellagio is a quick, easy ferry ride from Varenna, no need for an expensive water taxi (if you can find one).