Well, if you also use Thermal Expansion, then it’s really quite simple… put a Liquid Transposer + two water sources opposite each other BELOW the steam engine, with the output side facing up. It will automatically fill the boiler and keep it supplied with as much water as you need.
Failing that, a couple of infinite water supplies plus pumps.
To my knowledge, like the Stirling Engines from vanilla Buildcraft, the Commercial and Industrial Steam Engines don’t accept any sort of coolant- they will only explode if too much energy builds up inside them. This could happen if the engine is powering a machine directly (i.e. not through conductive pipes) and produces more energy than the machine will accept, or it you’ve daisy-chained engines (i.e. one engine directly powering another) and the engines toward the back pump more energy into the engine at the front than it can output into whatever it’s facing. Or, if your engine isn’t powering anything at all, it’ll still overheat and explode.
So, connect your engines only to wooden conductive pipes (i.e. not each other or machines(except via conductive pipes)), and you shouldn’t have any problems.
And by the way, that would be the Aqueous Accumulator, not the Liquid Transposer.