I think that guidance is mistaken. I think the requirement in the US is to maintain a radio watch if the vessel has a radio. I don't think you escape the requirement by switching off your radio.And even in the US, I think it's a case of: Vessels not required to carry a marine radio (e.g. recreational vessels less than 20m length), but which voluntarily carry a radio, must maintain a watch on channel 16 whenever the radio is operating and not being used to communicate.
So it's not a requirement to have the radio turned on all the time.
Here is the language from the actual regulation, 47 C.F.R. § 80.310:
Watch required by voluntary vessels.
Voluntary vessels not equipped with DSC must maintain a watch on 2182 kHz and on 156.800 MHz (Channel 16) whenever the vessel is underway and the radio is not being used to communicate. Noncommercial vessels, such as recreational boats, may alternatively maintain a watch on 156.450 MHz (Channel 9) in lieu of VHF Channel 16 for call and reply purposes. Voluntary vessels equipped with VHF-DSC equipment must maintain a watch on 2182 kHz and on either 156.525 MHz (Channel 70) or VHF Channel 16 aurally whenever the vessel is underway and the radio is not being used to communicate. Voluntary vessels equipped with MF-HF DSC equipment must have the radio turned on and set to an appropriate DSC distress calling channel or one of the radiotelephone distress channels whenever the vessel is underway and the radio is not being used to communicate. Voluntary vessels equipped with a GMDSS-approved Inmarsat system must have the unit turned on and set to receive calls whenever the vessel is underway and the radio is not being used to communicate.
The coast guard also restates the requirement in plain language here: "In general, any vessel equipped with a VHF marine radiotelephone (whether voluntarily or required to) must maintain a watch on channel 16 (156.800 MHz) whenever the radiotelephone is not being used to communicate."
This issue actually comes up from time to time in kayak discussion circles (including here on WCP at least once, if memory serves), because, under a strict reading of this rule, a kayaker would have to run his or her VHF all the time while paddling, which is obviously unworkable for a multi-day paddle.
EDIT: Ah, I see the source of confusion. The original version of the regulation did, indeed, say that voluntary vessels only needed to maintain a radio watch "whenever the radio is operating and is not being used to communicate." Source: 51 Fed. Reg. 31249 (Sept. 2, 1986).
But the FCC is clear that the changed language was, indeed, intended to compel radio-equipped vessels to maintain a watch at all times while underway, not just while the radio was in operation. "Since the Report and Order clearly reflects a Commission intent that voluntary vessels maintain a Channel 16 watch whenever the vessel is underway (except when the radio is otherwise being used to communicate) and not just when the radio is being operated, we take this opportunity to amend Section 80.1153 to reflect that intent, conforming it to Section 80.310 and rectifying the earlier omission." Source: 21 FCC Rcd. 10291 (Sept. 8, 2006).
It seems some of the online, user-friendly, non-governmental guidance websites are still relying on the old regulation, which only required a watch to be maintained if the radio was switched on. That's no longer the rule. Always check the text of the law itself, not just what some guy on the internet says about the law. Present company excepted, of course!