We went on the Crown Princess Dec last year and its a lovely ship. Large but doesn't feel overwhelming. Food was great, staff were good in the main. Watch the tour desk people though they were less than helpful. Also I would not go for a cabin on deck 14 as you could hear noise from above. Hope this helps, Kind regard.
Weather conditions in August are liable to be good with flat seas but there is a chance of thunder storms. Most of the ports are easy to do your own thing provided you do a bit of study about them before you go but St. Petersburg is one that you ought to buy ships tours and excursions for because the cost of visas and the extra formalities and risks make it just too much trouble. Look at the princess web site and there you will see all the tours that you can do. It also helps you to sort out and study those bits you want to do on your own or with fellow passengers. Best place to wander on your own....Tallinn and very good beer in the square after.....you should love it....Falan Dow.
Crown Princess is a wonderfull ship. We cruised november 2006 in the Caribbean. We got married in the chapel and had a wonderfull tme. Just done A transatlantic on QM2. Not a patch on crown princess. Won`t travel with Cunard again. Sorry don`t know about weather conditions.