mixed weather is quite likely, it can be beautiful but not necessarily every day. Take layers and a brolly, just in case. Have a look at the "Related question" at the bottom of the page.
The Mrs and I went in August, and it was really a mixed bag. Some hot, sunny days, some cold, wet and miserable ones. It's not a cruise you'd choose if the weather was the main attraction.
I was there a few years ago in July - a day of rain, but not very cold. The other days were really warm. Don't forget to buy lots of amber, especially if you visit Palanga, where there is a fascinating amber museum, and street stalls selling beautiful necklaces, etc. You could buy presents for everyone, genuine amber, at quite surprising prices. And yes, if they tell you the green stuff is amber, too, it's true. White, and all sorts of other shades too.