Maps with water such as 2Fort, Well, and Hydro the water has the same physics as Half-life 2 being you are able to jump on the water but you regain your health after losing health then breaching the surface. You can drown in the water and your body will rise to the surface. The pyro can jump on the water to burn people who are underwater though. The engineers buildings function a little diffrently then they normally do on land, such as the sentry will only target people underwater and the rockets move more slowly, the dispenser heals health until it hits a certain point where it think you are at full health due to constant drowning underwater, teleporters normally leave people confused when they teleport underwater. Weirdly enough speed effects are affected when under water.