Documentation No. location never a USCG boarding problem
I had a documented vessel for 17 years, boarded three times by USCG in WA and AK, # was in E/R, no problem.
Also skippered with fleet of 10-15 GB's (depending on year), USCG boarded twice, same result.
BUT number was either decals sealed right onto the engine stringers or overhead cabin sole beams, or (in my boat) was on metal plate screwed on with phillips heads subsequently drilled out so not removable.
BoatUS quotes USCG regs as follows:
"The vessel must have the official number permanently affixed in block-type Arabic numerals of not less than 3 inches in height, preceded by the letters “NO .” on some clearly visible interior integral structural part of the vessel."
...note that no location is mentioned.