I do wonder what happened to their boat, though quite possibly it was something they had while their father was stationed in the UK and he sold it/passed it on to another officer when he was promoted and sent overseas, since there was no one to look after it. Thinking about Ransome, in We Didn't Mean to Go to Sea the children are told they'll probably have access to various boats that belong to the Navy/naval officers, so it may have been similar for the Marlows, and only 'theirs' temporarily.

The combination of country gentry types and Navy doesn't seem odd to me, though it's interesting that of the holiday books Traitor is very much about the sea and the RN, then they become much more countryside until Run Away Home which switches back again.