They never talk about the size or nature of the ship being serviced, but some of the container ships we worked on were 600 feet long, 120 feet wide, and drew 30 feet of water, and thats a small one. That is a lot of real estate. Some you cant let the oil to cool much below the operating temp of the engine, or they would not start. Years ago, they had gas turbine powered ships, they never shut the turbine off, they would idle it down as low as it would go, the prop would spin about 2rpm.
Some chests are 8' x 10' and 5' tall. Ones I have worked on have bars over them, but they are mainly to keep large objects out. A lot of countries do not have what we consider to be LOTO procedures. A lot of ships are registered in countries of convenience, and manned mostly by semi skilled crews with exception of captain and chief engineer. Turnaround time from arrival and departure may be 10-12 hours.
So, you show up for a job. They walk you into the ship, down a couple of passageways, down 3 flights of stairs, into an engine room 80 feet long, show you the intakes they need to keep hot. Then, you go out of the engine room, up a few fights of stairs, through a couple of passageways, back out on deck, and try to pace off about where the chests would be located. None of the crew knows exactly, they have never seen the underside of the ship. Bow thrusters and stern thrusters are different, they are usually marked. We did a lot of work in San Juan. At high tide, we could see 40 feet. Low tide, 5. If you are welding zero. Your supervisor and tender try to keep you away from the intakes.
Locks, damns, and powerplants have their own problems, however, if you think about the amount of work that goes on worldwide on any given day, and work for and with people you trust, its reasonably safe.