they are supposed to be completely sealed from the elements (buy a quality kit -- Streetglow). I have noticed some condensation in one of my tubes before though.
I used to drive my old beater car through high water areas. Now that I have the drop and alot of wiring going on under my carpet and a few extra holes in the firewall, I tend to stay away from high-water areas and not drive though them.
There is a very convenient place to mount the tube to the frame in the front such that it is above the lip line of the bumper. To destroy the tube, you would do serious damage to your bumper first. As you can get a replacement tube for $50 shipped (Streetglow pricing anyways...) the tube is probably the least of your worries.
Front and back tubes are well protected. The sides need alot more care especially if you have a drop as they can be broken quite easily if you straddle a hump (ie. drive over a curb, drive off the shoulder onto a lower section, etc...) Also, you have to worry about lifts at service shops as well. If you have a very significant drop, some places won't take your car since it can't get on the lift. One of my tubes was destroyed when a guy backed it of a lift once. It went up fine, but caught something when he drove it off and it shattered.
Good luck...
