To be pedantic, no we, the taxpayers, do not own the Steam Packet, Treasury does.
Yes, taxpayers' money has been used to fund the purchase and loans. But, in a way, that is no different to a bank taking deposits and those deposits being used to make loans. The depositors do not own the loans. I do get your point, but the real point is that Treasury should act as a shareholder and lender bearing in mind its obligations to the taxpayer to safeguard public funds.
As for how much the government is into the company, if you are including the Liverpool landing stage, that cost is nothing to do with the Steam Packet. It is an overall cost of shipping links to the island, and was not asked for by the Steam Packet but user costs are for it to meet.
That's pedantry!