Probably the reason some vendors do not give a max current specification is because there are different types of current meters in use that will give significantly different answers depending on how the meter is designed. The so-called true RMS current meters will only give the right reading under certain conditions. An analog panel meter may give a much different answer. Current input is also determined by input voltage (lower voltage causes higher current). Thus vendors may choose to avoid this confusing state of affairs by simply not giving the current specification.
Dockhead, if you are concerned about the current draw then that will sometimes be unrelated to the efficiency. Current draw can be considerably affected by power factor. A sometimes misunderstood fact is that low power
factor does not usually contribute to low efficiency. There are many snake oil
salesmen selling power factor correction devices as a means to vastly improve efficiency of equipment
Efficiency is simply the ratio of power out and power in. Power is the average of the product of current multiplied by voltage in real time. Unless there is effective power factor correction this is not the same as taking a voltage measurement and multiplying it by a current measurement.
It can be difficult to measure the power input to a non power factor corrected battery charger because it requires equipment
that most boaters do not have on board. A clamp meter and a volt meter will not give accurate results. A real time watt meter is about the only way to measure power into one these devices.
Sorry for the diatribe on efficiency and power factor but it is an oft misunderstood subject.