I mixed this book with the one where they go to camp(?) and learn about the ocean. Also I thought Benny hurt his foot on the beach as a catalyst to... something. But that might have been a scene from another book I read in the 90s (Baby-Sitters Club?).
Something that does bother me about the Boxcar Children novels is the adherence to gender stereotypes. Whenever there's cooking or cleaning or tidying it's always Jessie and Violet doing it. It's just assumed they'll do these things. The authority figures are always men (the detective they happen to meet, the store owners, the helicopter pilot). I hope women are given more to do in the Boxcar Children novels being published now.
Spoilers under the cut.
In this book Benny and Henry laugh because it's so absurd that two mystery figures on the beach at night could be women, but joke's on them. It was two women! (Seriously, that's part of the twist in the solution?) I definitely read them as lesbians (they live alone together with 10 cats... come on), but I'm pretty sure the word lesbian has never ever shown up in a Boxcar Children novel.