None. Paper charts and your eyes.
casailor 53 and rhans:
Given that boat chartplotters and backup apps on devices also require the use of "eyes", what is your rationale for not using electronic assistance in addition to paper charts? I
n your land based vehicles, do you use GPS units and/or cell phone apps or do you rely solely on Rand McNally atlases and road maps? If you use electronics in your car / truck / motorhome, then why not do so on the water?