Then she'll ask...
"But what about all the money the boat cost?????" 
Then I'll tell her that the "true cost" of boating doesn't include what we paid for the boat.
Nope, it won't work for her, and it won't work for me either.
The only reason the "cost of ownership" number is important is that a surprising number of people only look at the cost of the boat, and figure once they've bought the boat they only "have to change a tire now and again." They are totally unaware of the ongoing cost of keeping a boat operational.
I don't think anyone is making light of the cost to buy the boat in the first place. At least I'm not. You're absolutely correct, it's real money and the amount of it, whether cash or financed, has to be taken into account in determining if one can even afford to get into the hobby. Or if one needs to downsize their dream to match financial reality.
But the potential buyer also has to be aware that it costs a lot of money to own and operate the boat once they've figured out how to buy it. And that often--- according to the brokers I know--- is a cost that a lot of buyers, particularly first time buyers, overlook. We did, initially, and we were very grateful when our broker brought it up and gave us the kinds of ownership costs we'd be looking at.
When a newbie buyer is told that insurance is $1200 a year and that moorage is $4,800 a year and if they need to have the engine mounts changed it's going to cost $2,000 to $4,000 to do it, and the cost to replace that tired old exhaust hose is $1,000 and so on, it can be a real shock..
Sure, some of these costs can be mitigated by doing the work yourself. But a lot of new buyers haven't a clue how to do any of it, at least not at the outset.
So of course the purchase and finance costs have to be included in the overall cost of buying and owning a boat. But there can be a helpful reason for splitting the purchase/finance cost off from the annual ownership cost so a potential buyer has a better understanding of what they're really getting into.
We've seen several cases of people buying cruising boats (and sailboats) and as the bills roll in for fixing this or doing that they get themselves underwater with the boat and have either ended up losing it to the bank or having to sell it at a loss just to get away from the ongoing ownership costs. And I'm sure people on this forum with way more time in boating than us have seen more of these cases than we have.
That's the only value of calculating and talking about ownership costs that don't include the purchase/finance payment costs.. To prevent someone from getting in over their head even if they can afford the initial acquisition cost of the boat.
