You'll find all sorts of references to tying down safety valves in the
old literature on river boats. That was also the pressure gauge,
pre-Bourdon tube. The Smithsonian used to have some of the old stuff
on display, the valve was usually a plunger and lever with an
adjustable weight on the outboard end. The example I saw was pretty
corroded, I imagine, though, that there were markings on the weight
beam for boiler pressure. Life could get pretty exciting for the
boiler room crew if the plunger stuck.

No boiler level water-glass gauges, either, just try-cocks.

Stan