EWS - Engineering Watch Supervisor
The EWS is the senior enlisted watchstation on watch. On a submarine, the nuclear watch section usually consists of the EOOW (Engineering Officer of the Watch), EWS, RO (reactor operator), EO (electrical operator), TO (throttle operator), ERS (engineroom supervisor), ERUL (engineroom upper level), ERLL (engineroom lower level), and AEA (auxiliary electrical operator). Additionally, depending on the configuration of the ship, there is either an ERML (engineroom middle level) or AMS/AMR UL (auxiliary machinery space/room upper level), and an AMS/AMR LL (auxiliary machinery space/room lower level). Finally there is the underway ELT.
The EWS is trained to know the job of every other watchstander. Normally, the EWS makes periodic tours of the engineering spaces and reviews the operating logs (written records of equipment operating conditions/parameters). The EWS and EOOW can exchange places as supervisor of the maneuvering room. The EWS can substitute for any watchstander on a short term basis, except for the reactor operator.
The maneuvering room watchstanders are the RO, EO, TO, and EOOW. The RO is the senior ET and the EO is the senior EM. If I recall correctly, the RO must be stood by an ET. Both ETs and EMs qualify as EO, but the EO is normally the senior EM. The TO is almost always an EM. These watchstations monitor and control a single panel and are not allowed to leave their position. The RO has direct control of the nuclear reactor. The EO controls the electrical generation and distribution. The TO controls the speed of the ship.
The MO (mechanical operator or MM) watchstations are ERS, ERUL, ERLL and, where this space exists, the AMS/AMR LL. I'm probably forgetting someone because I have blocked out the experience of serving on a 688i class submarine as much as possible. The ERS is the senior MM and makes roving tours of the engineering spaces. The ERUL, ERLL, and AMS/AMR LL are restricted to their level, but are allowed to wander the length of the space.
The ERML or AMS/AMR UL watch is another ET/EM watchstation normally stood by an ET. This watch is restricted to the level, but is allowed to wander the length of the space. (This might not be completely accurate for the ERML watch, I forget.
The AEA is another ET/EM watchstation normally stood by an EM. The AEA is generally confined to the engineering spaces, but can possibly go anywhere on the boat.
The ELT can either be a dedicated individual who is on call at any time. The ELT can also be an off-watch duty for an ELT trained mechanical operator.
Further descriptions of the operating spaces and equipment border on being classified information. Plans and descriptions are available in books. Actually, the equipment configuration is probably not classified except for certain equipment, but I don't have the governing instructions, so it is best to say nothing about the subject.
When the nuclear reactor is not operating, the manning is reduced. There are as few as one shutdown reactor operator (SRO), one shutdown electrical operator (SEO), and one shutdown mechanical operator (engineroom roving watch). The EWS stands duty as EDPO (engineering duty petty officer) and the EOOW becomes the EDO (engineering duty officer). I'll stop at descriptions here because the SRO in most posts refers to a civilian position, senior reactor operator, which is completely different.