Airboat

http://www.halflifeportal.com/info/images/airboat.jpg

This is kind of like "the skiing air boat" of HL2, with a machine gun front mounted, it's a great vehicle for covering a lot of distance in water and dangerous hazard water filled areas of HL2, while also maneuvering quickly and avoiding enemy fire / damage.

A pulse cannon is mounted on the front by a vortigaunt and is useful for clearing your path of Combine soldiers and the apache helicopter.

http://www.halflifeportal.com/info/images/airboat.jpg