How do I get past the bridge/dam in the chapter Surface Tension?

By this point you should already have received the Gluon (right click charges up, then release to "insta" kill).

What I did was, when you first see the soldiers going towards the dam, I waited until they actually passed the gates. Next, I ran close to the entrance, where you can find two health kits. Then I fired a quick shot or two to trigger the aggros.

Then I ran back up the road to a fenced up power generator area, and camped a corner, equipping the shotgun. As the soldiers came by, I would use the shotgun's secondary fire to finish each one quickly.

When you are at the entrance of the dam, you should see to the right a warehouse. Enter inside, and you should find some Gluon Ammo and some other ammo packs. At this point, after you exit, a helicopter should come trying to murder you. While normally you can't destroy it, using the Gluon can incapacitate it. Otherwise, you'll need to jump into the water (beware the monsters)

As for entering the water, if you got the Gluon, it should be a piece of cake. Just charge and fire, or spam and fire.

After this, in the middle of the water, is a guard tower. There should be a ladder, just go up, press the button, and you'll know what to do!

As Howdy_McGee mentions below in the comments:

Update on this - You can't fire it underwater. It just keeps clicking like all the other guns. You can charge it above water and fire it under water there but that's it. Either way got past it when I figured out where to go and playing it on Easy.