I can address some of the tug/barge work schedules here. Now, my info is a bit dated, but some still applicable. When I first started working on ocean tugs, about a year after graduation, it was for a major ocean towing/transportation outfit that is even larger now. We ran a regular run from Louisiana to Puerto Rico. I was an engineer, so that meant that after my training period, I WAS the engine department. I stood the 8-12 watch each day, and left the machinery space "unmanned" the rest of the time. My cabin was adjacent to the engine room in most boats, and I would also respond to all alarms and other requests from the deck department or the cook.
The deck department on those tugs worked the three watch system, with the Captain on the 8-12 and the other watches split between the Chief and Second Mates.
When I left the cushy trailer barge run to the Caribbean, I then moved to an integrated tug/Barge; one of the earlier, if not the first Articulated tug/barge design. This vessel was operated by a Scottish ship management company and was their only domestic US flag vessel. In the engine room we had the Chief (me) and an assistant. It was my option as to whether we stood one twelve or two six hour watches each day. I ended settling on 6 hour watches while underway and 12 hour watches in port. Not very effective, either way. The deck department was on the three watch cycle, however. I worked on this particular vessel for some time, and was able to get them to add another engineer and we worked the three watch system. Since we were attached to the barge (a petroluem product barge), there was always work to do in and outside of the engine room.
The trend that I have seen for all of shipping, and not just in the US, is the reduction of crew costs. This can be achieved by reducing the number of personnel onboard the vessels, as well as by cutting their pay via outsourcing the crew and officers to nations where there is a lower per capita income. US merchant seamen faced outsourcing long before their onshore counterparts.
The first couple of ships that I sailed on had crews of 35-35 personnel. Now, many ships get by with less than 20. On the ITB that I sailed on, we had 12 and that included two non watchstanding tankermen.