All the BOBs on this level are exploding. Kill em all.
Avoid hitting the S'pht'Kr. Their shots (as you can see) are QUITE nasty, and you'll have little chance of defeating them. Fortunately, if you hit one, only that one will turn on you. Same goes for the S'pht'Kr on "Feel the Noise".
In the one area where you need to swim down and up to move between rooms, you may have a problem with BOBs lining the edges of the pool where you need to get out. To fix this problem, switch to the fusion pistol, and move as far from the edge as you can without leaving the room. Stop swimming, let yourself hit the bottom. Then, swim again, aim slightly up, and when you break the water's surface, fire at a BOB or more. Repeat until you have a clear break.
In the last room with the narrow ledges surrounded by water, there is a darkened square that can be seen underwater. If you stop on this square, you'll be teleported to a room with ammo and exploding BOB's. The ends of the room teleport you back out, so be careful to keep running there if you are still collecting ammo.
In the room with the large number of closed doors (7 or 8 of them), there is a closet with shotgun shells near the ceiling of the room. You can find the exact spot by look, at the surface of the water from beneath it and finding the lightened spot.
There exists a room on this level that is not obtainable via any non-cheating method. It contains a couple of switches that play with the water level, but nothing else apparent.
Marathon 2: Durandal - Level 26: Fatum Iustum Stultorum