Bobbsey Twins on the Deep Blue Sea by Laura Lee Hope

This is the 11th in the original series of books about the Bobbseys -- two sets of twins in one family, solving mysteries and having adventures. Bert and Nan are 12, Flossie and Freddie are six. There is a father who works, a mother who stays home, a cook, a handyman, and an assortment of animals. - Summary by Nan Dodge