Although this is a shoulder season, I would want to have a definite, physical ticket for the return boat before I got on the "to" boat, especially if I were planning on the last boat of the day. Also, be sure to check (probably can't do until the hour before departure) whether your return boat is going to use the SAME ferry slip in Capri that the arriving boat did. The return boat could be a different vehicle, or a different size.
Even in April, a day in Capri can be sunny and hot, and you can walk five miles easily. Many areas are completely pedestrianized and inaccessible to taxis, jitneys, or anything but feet. So make sure you think about your personal limitations. In season, it can be hard to get a formal meal without a reservation. But in April, it should be easy to get an informal lunch somewhere - just not in a main square.
Positano can take only smaller boats than some other AC ports and Sorrento. While I have NOT taken a Positano ferry, a vulnerable guest should have seasick pills ready, no later than when they see the actual boat waiting for departure. It's a long ride on open water.
I do not mean to dissuade you from the outing. Capri is beautiful and enjoyable, especially if you are ahead of the worst summer crowds.