Is the boat in Yiheyuan opened throughout the year or is there a closed period?

Generally speaking, the boats open throughout the year.
But before you go to the scenic area, you need to notice the weather. The scenic area will close the boat if the weather is bad.
Different boats have different service hours. In winter, you are suggested to tour around the lake by boat within 2 hours.

the travel agent we are booking with told us that the boat is closed till 1st April. We are travelling in March.