I used to work for the State Park Service as the GIS project specialist and occasionally had to check up on things like illegal trails reported to us by the public, which also usually included coordinates they pulled off their phone. My experience is that it was rarely very accurate - it would probably get you into the general area (within 30m or so?), but there were also times that the information they gave me was nowhere close to where I actually needed to be (the worst instance I can recall had me more than a half mile from where I was going).
I've found basic Garmin units to be reasonably accurate (10m or better consistently given ideal conditions), but they're still susceptible to being hampered by weather, line of sight with respect to satellites. etc.. The units I used for work were sub-meter and sub-foot for comparison - potentially highly accurate, but still dependent, to an extent, on the environmental conditions.
I haven't personally used OnX. If it relies on the iPhone for the actual positining, I wouldn't expect it to be exceptionally accurate, but it should be "good enough" to get someone into a general area that they're probably already familiar with. If you can see the shoreline and make some notes to sort of triangulate your position and use in conjunction with the phone, you should be fine. I would prefer the Garmin over an iPhone, but if it's what you've got and you're not trying to locate something too small out on truly open water (think an extended drop off that goes on for 100 yards versus just three old stumps on a tiny hump), I wouldn't hesitate to roll with it.
