The driver pulls up in this old rickety WWII troop carrier that looks like a boat, that's called a Duck. This is because it's an amphibious vehicle. Everyone is kinda confused, but he hustles you on through the back (the only way to get in from the ground). The driver turns out to be a great guy, making tons of jokes while being serious about safety and stuff.
Then the tour starts. First they drive around downtown Baltimore, explaining the importance of several obscure, but very interesting buildings and memorials (such as Edgar Allen Poe's grave). Then you visit part of historical Baltimore, which is nice because you don't have to drive there (it's somewhat out of the way).
Finally, the fun part starts, and the driver pulls through an arranged boat dock and steers straight into the inner harbor. The "Ducks" are pretty steady in the water, and the waves in the inner harbor are not big at all, so everyone stays dry and has fun.
At the end of the tour, the driver pulls out of the water straight into the downtown pavilion area and drops you all off. It was tons of fun, and great way to sit down and enjoy Baltimore while also taking a break!