I've seen this figure put out there many times, but I don't think it's anywhere close to reality.
The slip or mooring cost is the same for a brand new $300K boat as it is for a thirty year old $30K boat. You can expect the repair cost of a brand new boat to be zero (because it's under warrantee), but at thirty years, you could be looking at an expensive rebuild or repower. So, maintenance costs can be expected to increase with the age of the boat.
Some boat owners are able to do some, most, or all the maintenance themselves, while others have to hire sombody to change a lightbulb.
My costs have never been 10% of the purchase price of my boat (yet).
I disagree with a lot of what Marin says, but on this one I think it's a reasonable rule of thumb. Think of the guy (or gal!) asking this - they have no basis so they're looking for guidance, and this isn't really that bad.
Sure you can get by with a lower percentage - if you defer maintenance, do stuff yourself, or I guess if you overpaid for your boat! <grin>
And you can get killed if you buy an old trawler and end up replacing the fuel tanks, stringers, encased plywood superstructure, cored decks, and so on. Or you can saturate with CPES and hope for the best.
So, how do you plan for the unplanned? You budget for it, so that when it does happen you at least have the choice of fixing it or dumping it and moving on. If it comes in under 10%, well then you got lucky.
BTW, I wouldn't put much value in a (boat) manufacturer's warranty - they are going through bankruptcies and reorganizations so fast it's impossible to keep up. Equipment manufacturers, sure - they do a better job.
I have a small spreadsheet I have enhanced over the years to let me figure the net - net monthly cost of owning a house, boat, or car. My boat variables are: price, length (moorage), investment rate (forgone earnings of money tied up in the asset), loan parameters (going to have to finance that Fleming!), personal tax rate (possible deduction as 2nd home), maintenance rate, and depreciation rate. From those I calculate payment/equity/interest, tax savings, deprecation, lost interest, insurance (based on value), moorage (length), license (value), maintenance, and cash out (and net if financed or with tax savings).
I do not include enhancements - the sky's the limt on those. Just the dollars necessary to keep it in the same condition (improving some things, and probably letting other things accumulate).
It's true that my maintenance costs have tracked closer to 5% of the value, but I haven't done a good job of separating true maintenace from improvements (e.g. when I had the head plumbing reworked I added a holding tank). But I do a fair amount of work myself, and I happen to have an exceptionally valuable skilled assistant who works at a very reasonable rate - if I had to pay yard wages my costs could easily double.
I am also fresh-water moored under cover - I generally get by with a couple of coats of varnish on my rails with very little prep. And I can easily go 3 years between haulouts.
All up, I think that 10% figure is a reasonable approximation.