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.
If this is Half Life: Source, then you can start the game with this command line option in order to have it re-autodetect your screen and use the default size it first started with:
-autoconfig
This will not fix your config file, but it will ignore the current (bad) config files, allowing you to fix your resolution settings, close the game, then run the game without the -autoconfig option to use your newly-chosen resolution.
You can add command line options to a game from within Steam by right-clicking the game in your Library, choosing Properties, clicking Set Launch Options, then pasting in -autoconfig (or whatever command line option you're adding).
If this is Half Life (the original), then you can start the game using the -w and -h command line options to tell it to start up with a different resolution. For example:
hl.exe -w 800 -h 600
Will start the game up in 800x600 (4:3 resolution), and will allow you to fix your graphics settings to that next time you start the game without the -w and -h options, it'll use your new resolution settings.
Reference source: Command Line Options article at the Valve Developer Community site (thanks to 3ventic)
Best Answer
Position yourself towards the corridor with the hanging cables that are electrified. Behind you is a locked room with some scientists which you want to enter.
Turn towards the electricity. On the bottom of the right wall is a ventilation duct. Enter the duct and go to the right. You will enter the locked room. Watch out for the hidden barnacles. On the opposite side of the room to the right is a door, inside is a switch that will disable the power for that part of the map.