(1) Open the door and head down the first set of steps. An alien handler starts firing at you right away. Kill him, but not with a grenade or seismic charge; he's carrying grenades and you will want him to hang around. By this time another handler will move in from your left--grenade this one unless you are low on pulse rifle ammo since he only coughs up a rifle magazine upon expiring.
(2) Now move down the second set of stairs to floor level and head left (you will activate a chestburster in a canal here which can be ignored until the very end of the level). Shoot the first alien handler you see on the other side of the bridge you arrive at (he's toting more grenades, and if you want to you can lead him back to that first alien handler for convenience) and toss a grenade at the second handler. (At this point, you may wish to run back and forth between here and the entrance area to repeatedly reactivate the alien handlers with the grenades. You will pass that spot on your way to the exit door, and having a cache of grenades waiting for you just before you exit will be pretty convenient.)
(3) Now go across the bridge, ignoring the switch and door to your left for the moment since you need a battery before you can get in here. Now open the door on the other side of the bridge. As soon as you do so, an alien handler appears in the water behind you and starts firing. At the same time, coming for you full steam will be a pair of new enemies: quick-moving adult dog aliens! You will definitely want to toss a grenade at each alien to prevent it from being a nuisance in the future. Avoid hitting the barrels since this will kill them but not permanently and you will end up dealing with them again later on. Once they are dead, you can deal with the water-wading handler. Now detonate the barrels and shoot open the crates in the room. One of the crates in the right corner of the room holds a chestburster; the rest will yield 3 derm patches, a battery, and med kit.
(4) Before blowing open the row of crates toward the left, open the other door in this room. Inside you will encounter a ceiling-dwelling adult dog. The best way to deal with him is to dash away out of the room until he follows, then turn and start firing with the pulse rifle (you will have to aim up) until he drops. This ceiling-hanger is actually pretty slow compared with the warrior variety encountered in previous levels. [PSYCHEDELIC ALIEN HANDLER TRICK: Come back to this area to reactivate this alien. Rather than return to its normal appearance, it will only be visible as an orange splotch on the floor. Aim down and keep shooting to kill it (even walking on it causes damage!) and upon expiring, a red/orange/blue alien handler appears! He will not attack you, but instead walks in a straight line until hitting the nearest obstacle. The alien handler actually looks like what a Predator might see using its infrared vision!] Having killed this dog alien, grab the auto-map and battery within the room and turn your attention to the row of crates in the main room.
(5) Shooting them clears an opening for you but releases 2 'bursters. You can now hop into the water here (it will not hurt you but does slow you down), and stay to your right almost all the way to the end for a grenade. At the second corner there is a chestburster in one of the pipe outlets. He tends to get "stuck"--you can see him on your motion tracker as well as hear him, but you can't actually see the critter to shoot it. Try shooting around with the shotgun while aiming down to kill it. When you grab the grenade, you may reactivate the first alien handler on the level, and if you do he will wander around after you and wind up just about anywhere.
(6) Now work your way back to the room where some revived chestbursters will be waiting for you. Go out and across the bridge and activate the switch and door you passed up earlier. Kill the 2 alien handlers and blow all the crates and barrels; this will release 2 more 'bursters.
(7) Kill them and face the far left wall of the room. Between and below two small lights near the ceiling is what looks like a door. It is. Toss a seismic charge at it and enter. Inside is a cylindrical lift in the UP position with a med kit lying behind it. Face the med kit. Run over it to activate the lift and run out of this area into the main room. On top of the lift and now hot on your tail are 2 adult dogs. Grenade them both and then return to the now-lowered lift for a seismic charge.
(8) Exit this room and head toward the entrance area. You will now want to take your first left. Ahead is another grenade-carrying alien handler emerging from behind some crates and barrels; a second handler will follow shortly. Detonating the barrels and opening the crates releases a single chestburster; I recommend grenading it permanently out of the picture or else it will stand between the only armor on this level and the exit door later on.
(9) Moving forward brings you to another stack of crates and barrels. These hold 2 more 'bursters for which I also recommend the grenade treatment. Opposite them is a row of crates blocking entrance to a room. Shoot them open and grenade the 'burster.
(10) Now enter cautiously. Your mission here is to successfully grenade the 2 adult dogs inside. If you fail and one or more revives, it will pursue you across lifts and through water to get at you later.
(11) This accomplished, head back toward the last stack of crates and barrels you blew to a switch ahead. Activate it to raise a lift panel in the water to your left. Now grab your mask and snorkel and hop in! The lift you land on will rise up even with the one you just activated, allowing you to move forward to another room.
(12) Ignore the next switch (you will use it on the way back) and stay to your right till you dead-end at a large switch panel. Activate it to open the exit door.
(13) Now turn 180 degrees and stay right; prepare to get wet again! Jump right in and stay right to the junction. You have 3 canals to explore--call them LEFT, CENTER, and RIGHT.
(14) The shorty to the LEFT ends at the bridge you crossed earlier and contains nothing (any alien handlers in the vicinity not vaporized earlier will reappear now).
(15) The CENTER waterway leads to a room blocked by a wall of crates. First, kill the chestburster waiting for you in the pipe outlet at the end of this canal. Now stand back and open the crates. They hold 3 or 4 chestbursters (it varies), 2 seismic charges, and night vision goggles. Lying on the floor behind is an adrenaline burst.
(16) The RIGHT canal has a derm patch lying in the middle of it right near the beginning and 2 'bursters hiding in a pair of pipe outlets at the very end.
(17) Having completed your explorations, head back to the room where you jumped into this canal and activate the switch you left untouched. This raises both of the lift panels you used to get here when you first entered the room earlier.
(18) Now move back past the first lift panel switch and head toward the entrance area. Near the beginning, where you encountered the first alien handler, is a canal with an opening right in front of a lift panel. In the water is a chest burster that was activated much earlier but ignored until now. The easiest way to deal with it is to step into the water and onto the lift (which immediately rises). This will allow the chestburster up to floor level where you can dash down the corridor and, turning, grenade it. The purpose of grenading the 'bursters is to have an enemy- and acid-free path between the suit of body armor near here and the exit.
(19) Before grabbing the armor, though, you have a few more chestbursters to kill. Stay to your left around this canal until you reach the exit door (the hall leading to it is dead ahead, with the canal to your left, if you stand with your back to the second set of stairs leading down from the entrance doorway). Shoot the five crates one at a time; 4 contain chestbursters which require the grenade treatment.
(20) This done, move back to the lift where you let the chestburster up, jump in, and grab the body armor at the end of this canal.
(21) Go back to the lift to get out and make your exit with a full 200 armor points.