Yes a lot of hotels have boats that go across as well as some public ones. The hotel i stayed at Mandarin oriental had their own one. You can ask your hotel once you get there or send them an email.
If you followed what he said then you would actually be buying the motorbike under his name but he would be allowing you to use it. A lot of motorbike shops do this with foreigners. They are mostly not trying to cheat. They are trying to help you out. Lots of my friends who trust me have also putten their bikes under my name as i have thai id. You could also put it under someone you trust with thai id or residential certificate.
As someone with both thai and uk passport. If he entered Thailand on his Danish Passport, he needs his thai passport to leave Thailand, just get it renewed at the passport office with both mother and father.
I had a problem many years ago regarding this. They managed to freeze my account from me and withdraw my money. I had to threaten them with some dangerous friends to get the money back.