Why is insurance not included in the rental? Full coverage, how much more could it cost?

Because it would cut out the scams.

A search on Aliepress (.com) yields a new 1300cc jet ski can be purchased from China and shipped to Thailand for US $5,848.14 or about 179,000 baht.

Do you also drive a Chinese car or Chinese Motorbike ? lol, These Chinese things differ a lot of that what we are used to!

I disagree. The Germans now manufacture Mercedes motor vehicles in China.

Yes, possible, German technicians have designed something and it is produced under German supervision. BUT,

above discussion, is about Chinese products,

which Chinese technicians have designed, produced under Chinese supervision.

These products have than no Mercedes *Star* to show!