"In your case, if you had a representative photo of your or a similar boat, and photoshopped out the logos, and sail numbers, you could use it and caption it with the boats name." Actually that's quite dangerous. A major newspaper got caught a couple of years ago when a staff artist "reinterpreted" a piece of someone else's work as part of a composite photo-illustration. IIRC they took a hand (literally) from a full-sized work and did some heavy manipulation on it, so the result was very different from the original full work. They wound up paying six figures for that work.
If the author can recognize enough of their work to say "That's from my work" you are at the mercy of a jury, and you'll have to pay legal costs on top of that, win or lose.
I don't find much gray area in the copyright laws, but that's also because I'm trying to read them simply--not looking for weasel paths.
"but it's always best to be safe." Gotta agree with you there! The intent of the Berne Convention (international copyright law) is pretty simple. Anything that isn't expressly permitted, is forbidden and belongs to the author. When you read the laws with that in mind, they become much simpler.
Only thing I haven't found a good reference to, is US public copyrights. That is, works of the US government belong to "the people". So those of us who are citizens own (share) them and can use them freely. But I can't figure if that means a Brit or any other NON-citizen can use them without permission. The Copyright Office doesn't get that explicit on their web site.