"Smartphone apps are too small."
Speaking of small things, cell phones ALWAYS used to have a little hole where a lanyard or wrist strap could be attached. But the companies quickly caught on to the fact that they could sell a lot more replacement phones a lot sooner if they deleted the attachment point and made it impossible (literally, impossible) to put a wrist strap or lanyard on the phone.
Note that anything small and valuable stands a pretty good chance at going overboard unless you are clutching it tightly, or there's a way to attach it. Cell phone, handheld VHF, bottle of sunblock...especially if a wake hits you, unexpected, and the boat starts rocking.
"DSC or not, I thought it was illegal to use a marine VHF over land. "
It is illegal, unless you've got a special license, i.e. for a yacht club's dockmaster on the docks. There are plenty of other radios designated for shoreside use.
Five horn blasts is not interrogation, it is the danger signal in US Inland Waters.
Boat US - Online Boating Safety Study Guide
BoatUS thinks so too:
"•Five Short Blasts - This is the DANGER signal. Remember, that when you approach another vessel and hear either one or two short blasts, and you both understand their signal and can safely let them do it, then you are required to respond with the same signal in response. However, if you don't understand their intentions, or feel that their proposed maneuver is dangerous to either vessel, then you are required to sound the DANGER signal."
And I'll tell you that I've heard it used by unschooled amateurs like the NY Staten Island ferries.
"Five" signals are actually common in the US. Five bells means "turn out all available equipment" to a fire company, a "five-alarm fire". Five bells on the old news teletypes meant "Drop everything the world just ended". Used when NORAD accidentally declared a nuclear attack had started, and when JFK was shot. Five bells is never good news.