Not for boat rental but for in door eating I would play eat safe and call ahead!
For lunch during the week and dinner at any time, reservations are recommended. You could get lucky, but it can also be very busy. They do not take reservations for brunch at the weekend - it is first come, first served. Wait times for brunch at the weekend can sometimes exceed an hour, but you can have a drink at the bar while you wait or walk in the park nearby (you don't want to go too far or the buzzer they give you will be out of range). Not sure when they shut down the boating on the pond, but that is fun to do while you wait too and the buzzer works on the on the pond! Whether you go for lunch, brunch, or dinner, it's a great place to eat and soak in the park and city.
Get quick answers from Loeb Boathouse staff and past visitors.