Temp gauges are hardly "useless". Like any guage however, they need to be looked at to see what's going on. You already have an audible alarm that comes on at a pre-determined point which is pretty close to the point where the engine gives up. So why not install a temp gauge so you can actually see the temp rise or any reading that is abnormal. A water pressure gauge can be installed on most engines. The degree of difficulty varies but most operate very nicely via a simple "Tee" fitting in the telltale hose. The difference between water pressure gauge and a temp gauge is that the pressure gauge indicates the water pump is working, not working, or working but not up to par. Just because it is working does not mean you won't have an overheat condition. A stuck bypass valve, a stuck thermostat, a head gasket issue, ignition timing issue can all cause overheat. An overheat obviously will occur if the water pump is not working. As for useless gauges, try a trim gauge when you can turn your head and look at the engine. If you can't see it, THEN it becomes a usefull gauge.
Here is how a pressure gauge is fitted on a late model triple or four cylinder Merc.
Here is a water temp sender installation on the same series engine.
And if you don't have an unused port into the water jacket of the engine for a threaded sender, you can use this type that mounts under a head bolt.
