Weston Water Treatment Plant is a location that consists of a large area occupied by super mutants and a large building. Try to eliminate the enemies from a distance, you have no chance in close combat. After you pump all of the water out, you might want to explore the pumping station again, because some places will now be available.
Most of the building is flooded. To pump out the water, you have to activate four switches. While you explore the building, watch out for Mirelurks and turrets in the rooms.
1. Exit to the roof. You'll find a safe (Advanced) and a terminal that can open it (Advanced as well). You'll find some ammo and medicine inside.
2. A safe (Advanced). Nothing valuable inside, aside from a Stimpak.
3. An elevator going to the pump rooms.
4. The first switch.
5. An expert locked safe is here and it contains ammunition. You'll also find a Picket Fences magazine on a nearby desk and picking it up will unlock new items to be built in settlements.
6. The second switch.
7. The third switch.
8. The fourth switch.
9. Main pump switch.
You might encounter a Legendary Mirelurk while exploring the building - it always drops one legendary weapon.