Visible crew/equipment: When Susan jumps from the inverted table to the curtain held taut by the men below, you can see the shadow of the microphone on the curtain and defined near the top. (00:38:00)
Visible crew/equipment: When Rev. Scott swims up the first stairwell, after pushing aside the dead crew member, as his head comes up the shaft, you can see, very quickly, the flippers of a crewmember swimming out of the shot behind him. (01:22:55)
Visible crew/equipment: During the dining room scene when everyone is eating. Look for the shot where Shelly Winters reaches over to Manny's plate, in the background you can see a boom mike.
Visible crew/equipment: When Belle is swimming, you can see at one point, that she is swimming in a tank, with a crew member in front of it guiding her with a flashlight to turn right.
Answer: Because he doesn't boil, there's no suggestion that the water is that hot. After Ackers falls in, and while Rogo tries to save him, there are two large explosions in the shaft; we can presume that, while Rogo was able to escape, Ackers was injured/trapped/otherwise incapacitated below the surface, and drowned.