Easy to rent, they speak English, half day is enough. For normal bike it was 10 eur. Lock included, free toilets nearby. Biking roads start right there.
Renting the bikes was smooth, but not a lot of services included. We were not offered helmets and we had to ask for anything additional, like a phone support (which even cost extra).
The opening times for this listing are wrong! We trekked into Vannes with a long walk from the campsite and then a bus, only to find it closed, along with all other bike hire places in Vannes - this is frustrating, just change the opening times for this pin, super simple.
Rent your bikes from somebody else! We turned up at the quai as instructed in the email, but could not find the shop. After some time searching, we asked a shop owner who said that she gets hundreds of people asking her the same thing and said we should ask at the tourism office. They said you have to call because there is no physical shop, so we called multiple times - no answer. Eventually we gave up. After two days of waiting, we finally managed to speak to the owner on the phone, only for him to be overly aggressive, blaming us for not being there exactly on the hour and shouting on the phone. At no point had we ever been told that there was no physical shop and that we should expect a guy to just be standing on the quai somewhere with two bikes. He was so aggressive, he made my partner cry, offering no apologies or explanation. When I phoned back to complain, he was equally short with me.
This region of France is full of lovely people that go out of their way to be helpful and pleasant. Best advice, choose a bike rental that actually maintains their Google My Business account, clearly communicates instructions and doesn't bully customers on the phone!