Ah, San Marco, the center of tourism in Venice. It wasn't terribly crowded with tourists when I went, as it was winter and freezing, but I've heard it suggested that this is NOT the place to start your tour of Venice, as the crowds of tourists may turn you off of the city.
That said, the Basilica di San Marco is a gorgeous church. I can't tell you how much my feet enjoy walking on marble.
In the high season, the lines are said to be quite long, so making an early morning visit is probably your best bet. If you have a backpack, you can't bring it inside. I have a small daypack, though, which they let me take in.
When you enter, if you go up the stairs to the right of the entrance, you can pay 1,50€ for entrance into a little mosaic museum. This is definitely a must. You get a great view of the mosaics, as well as information and samples of some older incomplete pieces. The original horses are also on display in the museum. You also get access to the balcony above the entrance, where the horses are on display, for a nice little view of the piazza.
There are free bathrooms in the museum, but it's one per sex, so there might be a bit of a wait.