At first, strip off all your armor and weapons. They aren't of much defense at 1-st level, but slows you down cuz recovery.
Enter, get the chest at your right.
Jump into the water and swim right, to the door of room with study key. Pass the skeletons in room with broken wall in moving phase of TBM. Stand as close as you can to the door of key room, enter TBM, save. Open the door, in moving phase go in and grab the key. Imho, you can move further using sidestep, not forward move. Kick the air in opposite of frights direction in fighting phase, in next moving phase go out and close the door.
Next will be chest behind the table in NE room (see the map, Ribannah provided). Go close to the door (passing skeletons in TBM move phase). TBM, quicksave. You have 4-5 moving phases of cycle of combat to enter, loot the chest and leave; zombies have no perception at all and they are extremely slow, but they can open the doors. If you are doing this right, you are in corridor unnoticed.
Go back to the water and swim to the chest, left from the entrance, guarded by fright. Keep as left as you can, there will be some owerhanging floor cap ower you and you can hide from guard. Near the chest, go into TBM, jump out of water, loot the chest and kick the air in pool's direction. You have only one moving phase to drop back in the water unnoticed.
The next will be underwater chest in NW corner of pool. Dive in it's direction, keeping pressed crouch key for the basement of ledge, leading out of water. Loot the chest, return to direction of empty now chest, guarded by fright. Equip the best weapons and armor you found, we are going to kick someone.
You can see the empty hall with fallen table, swim to it and jump out of water and run forward to the door to room with four columns and zombie inside; open the door, run in, close the door behind you (fright cannot go in) and dance with the wolwes, er, zombies, around the columns. When both of them are dead, go to the study, in TBM moving phase loot the book and fifth chest, leave the room and all dungeon. The way out will be free - guarding fright still will be near the door you entered column room, and the south door with way out will be ungarded.
If you found the wand of poison cloud, the fright in study can be cilled with one shoot at it's feet.