USCG approved means it has been tested and meets the visibility distance requirement. if it is not an approved type then you can not prove it does meet the requirement. someone hits you and the lawyers find out you will be liable
Right. The bottom line is that you should have a proper marine anchor light that you're willing to use. When you start talking about using a $5 home depot light, that may or may not last an entire night, for a fairly important function you've got a problem.
In this case $40-$100 for a marine LED light solves that and it's worth it. I mean hanging battery lights are fine, but why bother? Wire in a permanent LED anchor light (the OP already has an existing one) and you're done.
EDIT: Funny story, when I bought my boat the previous owner casually mentioned that the masthead light didn't work so he climbed up the mast every time he sailed at night to mount a battery one. "Be careful" he said (at least the boat does have steps).
Needless to say one of the first things I did was debug this problem. At the time my electrical consisted of one + bolt , one - bolt and a rats nest of corded wires and inline fuses. Eventually I replaced some fuses and figured out that the masthead light was labeled "windshield wiper" on the panel. For the first season "windshield wiper" was my masthead light.
So that's one extreme. Personally, after re-vamping the entire electrical I'm thrilled to have a switch I can flip to turn on the new LED anchor light - no hanging or mast climbing required. (As a throwback I labeled the unused circuit on my electrical panel "Windsheld Wiper")