Toughbooks (and other so-called tough laptop/notebooks) are indeed somewhat durable, but they are by no means strong. If you dropped the machine in the water, the only thing that's keeping it from water contamination is little plugs (which can get lost). There would be no heat vents for this reason, so while this type of machine would withstand a drop better than even a Thinkpad and be durable form damage, it would not be durable in longevity (heat kills). Also, they are way overpriced and are a little bit behind the times as they don't have the latest technology. It would not be something that you'd want to take to meetings or in a cramped airplane. If you want reliability, Lenovo Thinkpads are the way to go. Toughbooks would be good only for military apps or for people who are out in wild nature who need computing convenience.
My suggestion: Lenovo Thinkpad R or T series.