Well Jane, I did the same thing a few years ago in October 2013. I had been in Portovenere for lunch exactly 50 years earlier when an exchange student in Italy, and thought it was the most beautiful place I had ever been. I had only a vague memory of this, so this became something of a pilgrimage. The sea looked calm to me, but I was told it was too rough for the ferry to go. Maybe on the return trip. So I took the train to La Spezia, walked several blocks to the bus stop, waited for the bus, then took the bus to Portovenere, about 90 minutes in all. The town is pretty, an old church, an abandoned castle as I remember, an interesting cemetery. Some shops and restaurants. But then, no ferry to return either, so I repeated the trio by bus, walk, and train, another 90 quite boring minutes. So, my guidance: if the ferries are moving, go to Portovenere for the day. If not, very possible in April, have another wonderful plan B, of which there are so many in that area.