Didn't see many boat threads but we still ended up with one somehow
And looking at the amount of posts it gets I would say, in my opinion of course, that that is a mistake.
Forums should only add new sub forums when enough posts crop up on the subject in the general chat to warrant one. By splitting them up you reduce the amount of people looking at them unless there are plenty of threads on the subject.
EG, when I come on the site I check out the console forum, phones tablets, books/tv/film, motors, motorsport and GD. If someone were to ask a question about a motherboard then I'm almost never going to see it, but fortunately there is plenty of traffic in the section and it gets answered and a thread continues.
If there was a travel section I probably wouldn't check it and so would never see someones question about staying in Menorca for example, or what a cruise is like. Since there are not many threads on the subject in GD, its not an in demand topic. Making its own sub forum would almost certainly end up with people asking questions and getting threds of maybe 5 replies because there isn't enough traffic rather than 25 because its on GD and lots of people see it.
Subsection with no traffic = graveyard, that means people either post there and get few or no answers/discussion or end up posting in the GD anyway just so that it gets seen.
That's all my opinion of course, but I've seen so many good forum communities go from fantastic (eg 4 forums) to dead (25) because the people running it expanded way too fast trying to have a sub forum for every possible thing when there was no demand.
For the most part OCUK gets it right I'd say.