story identification - YA or MG novel from 60s or 70s (or earlier?), undersea habitat attacked by "vandal" submarine

I read this in the late 70s or early 80s: kids living in an undersea research habitat with a companion Grampus griseus; their parents are away for some reason (?) when the habitat is attacked by a submarine full of "vandals" (pirates?). A servant or support staffer with the leitmotif song “Lillibullero” turns out to be some kind of government agent and helps save the day.

Answer

This would appear to be "Secret Under the Sea" by Gordon R. Dickson first published in 1960. The summary from goodreads is the following:

Why is his dolphin acting so strangely? Something must be wrong.
It is the year 2013, and Robby lives in an Underwater Research Station with his scientist parents. Most of the time he has fun exploring the ocean caves with the dolphin who is his favorite companion.
But something has frightened the dolphin, and Robby sets out to investigate. Then he finds the giant footprints. And he knows that something enormous and unknown is walking here across the bottom of the sea.

Although this only partially matches (and I haven't read the book to confirm anything else) I found this Yahoo answers post (I know :/) that seems to match your description and has this book as the accepted answer.

When I was a kid, I remember reading a book about a boy who lived with his father in an undersea research station. The boy's father was a biologist and had genetically engineered the boy a pet dolphin. He was also doing some research into some transplanted Martian lifeforms. Anyway, the father had to take a business trip and hired a bodyguard to watch the boy while he was away because there were political factions (I think that they called themselves the Vandals and were identified by the long beards they grew) who were against the concept of studying Martians on Earth.
The story played out with the Vandals taking over the research facility to kidnap the boy. The bodyguard, whose name was Lilliburlero (sp?), evacuates the boy but they wind up getting caught. However, the cavalry arrives and saves the day in the end.

This is the cover I found of the book.

This page seems to suggest this book is the start of a series with the other books named: Secret Under Antarctica, Secret Under the Caribbean, Secrets of the Deep.