We did a similar cruise 2 years ago in August and as it was not a fly cruise where you have to worry about luggage weight allowance, we took clothes for both sunny/wet days. It turned out we only needed to use the 'sunny' clothes as the weather was absolutely glorious the whole time. The weather in August in these countries can be like the UK - variable, so best be prepared. (can recommend flat comfy shoes if you are going to Tallin, all those cobblestones - fantastic city though!) Enjoy.