Hey mahcaeh, did one of these a few years back ending up at St Petersburg for a a few nights. Most of the trip the weather was very similar to the UK, lots of rain. The only sun we had was in Tallinn and a day in Russia. So waterproof jackets and layers as mentioned. It's certainly not freezing cold, and on the ship you don't really notice. It was one of my favourite cruises, and i will definitely do it again in the future. Hope you have a fantastic time