Very nice shots.
I would have to second the comment on leveling the horizon.
One major problem with photographing tourist destinations is the hordes of people and tourism stuff (busses, vendors etc...). You have a head cropped in half on one image. Others have tourboats that are uninteresting.
Your focus, exposures, contrast, and color are excellent. Technically, they are well executed.
Composition is made difficult by surroundings. Try getting closer and taking shots that are more narrow to avoid undesirable clutter, or perhaps better yet, try to make the people an important subject of the shot.
In the first shot, there is way too much water and sky that are pretty uninteresting.
Saint Petersburg looks lovely. Your photos make me want to visit the city, but as art, it is just a bit lacking.