In this very spot, it's located the ferry port of Stresa. This spot though it's not where the official ferries are located, which is a few more metres to the right. Here are located the unofficial taxi boats, which are the best options to visit the Isole Borromeo.
You'll likely be approached by the ticket sellers of these different companies, but they are not scammers, it's honestly the best option.
To be extra safe, you can use the one I used, which was very convenient: it's called 'CMA Group' and you can recognise them since they have a red flag on the port.
The ticket to buy to visit all 3 islands costs €15 + €1,50 which is the price to disembark on the islands (€0.50 for each island). The total cost is €16.50 and you can also pay by card. The ticket includes one ride to each island and the way back to Stresa, so 4 trips in total.
The first ride is at 10 am, and I recommend to take this one, so you can start exploring the islands at opening time.
If you came to Stresa with a car, near this spot there's a big car parking lot.