When entering Japan, a foreign yacht = vessel is required to have an "insurance" .
Details Re the insurance are asked for by the Japan Coast Guard Initial Paper Work (PW).
Once in Japan...the JCG never will ask for "proof ".
So ...read between the lines.....it's up to you what to write down @ PW if you don't have insurance.
In addition, it is very easy and cheap to take out a third party insurance once in Japan...for a 40 foot boat it is about 10.000 yen for a year. A good thing to have with all the outstanding fishing nets etc.
In a way it is "funny" as Japanese yachts are NOT required by Law to have an insurance....
As a rule in Japan, 1 person on board a boat should have a Boat License.
This doesn't count for a foreign vessel.
As long as you travel on your foreign flagged vessel: No Need to Have.
Funny again is...if you do have an overseas Boat License...it is NOT valid to navigate on a Japanese vessel...
Hence you have to study and sit the Japan Boat License test.( email me if you like details).