I've got some more ideas on the dam jump scene as well.
For pretty much all of the first game, Freeman is searching for a vehicle so he can finally get out of Black Mesa. Finally, in HL2, he has one (and it's got a stylish machine gun on it too!). And since he's just taken down a chopper, he should be totally in love with the airboat by now, and should be very unhappy to give it up.
From his perspective, the airboat is the only way out of here. If more choppers show up, there's no way he can outrun them, and climbing and swimming through marshes like he's been going though is very slow going on foot, the combine would have plenty of time to catch him. His best chance to get away is to try and get the airboat into some open river or even out to the sea so he can try and get out of the country (he doesn't know they've taken over the world). In this case, jumping over a dam doesn't seem like such a crazy idea. The dam isn't very tall, and he's done some jumps before. The water is pretty deep, so it probably won't kill him if he screws it up. I can see him going for it.
And then raging like crazy when a whole 1000 ft away is another block he can't get around and he has to get off on foot