I think the thought against garden lights is while they are white and all around, they do not (or may not) meet the brightness / visible distance requirements, escpecially all night long...
Agreed, and thank you for the citation. Though I will post the same section for boats UNDER 50meters here as well. My boat is smaller that yours it seems.
I would also like to add that my recommendation to the OP is to replace the bulb with a high intensity LED at the masthead and use that. LED bulbs last forever, use little electricity, and I've never found a solar light that was bright enough for me and stayed on all night. If he is concerned about USCG approval he should replace the housing and bulb. While USCG approval will go a long way towards proving that your light meets the requirement, it may not be enough in a court battle. A dirty housing, low voltage and many other reasons could make your light not visible. It could come down to witness credibility etc etc. Again though, the USCG cert would help, just don't expect it to give you a free pass from the other guy's lawyer on this point.
Rule 22 - Visibility of Lights
The lights prescribed in these Rules shall have an intensity as specified in [Section 8] of Annex I to these [Regulations | Rules] so as to be visible at the following minimum ranges:
(b) In vessels of 12 meters or more in length but less than 50 meters in length;
(i) a masthead light, 5 miles; except that where the length of the vessel is less than 20 meters, 3 miles;
(ii) a sidelight, 2 miles;
(iii) a sternlight, 2 miles;
(iv) a towing light, 2 miles;
(v) a white,
red, green or yellow all-round light, 2 miles
(vi) a special flashing light, 2 miles.
(c) In vessels of less than 12 meters in length:
(i) a masthead light, 2 miles;
(ii) a sidelight, 1 miles;
(iii) a towing light, 2 miles;
(iv) a white
red, green or yellow all-round light, 2 miles.
(v) a special flashing light, 2 miles.