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The quick answer

For an ocean-mad 4 year old, start with Meet the Oceans (a rhyming read-aloud), National Geographic's Little Kids First Big Book of the Ocean (real photos, real facts) and Who Makes an Ocean? for the big questions. Aqua Boy is a lovely one before swimming lessons.

Bedtime with an ocean-mad four year old is basically a nightly deep-sea expedition: turtles, octopuses, sharks, ideally all in one book. These are the ones worth having for a curious four year old: proper stories, real facts, and pictures you'll happily look at for the hundredth time.

1
Best read-aloud

Meet the Oceans - Caryl Hart & Bethan Woollvin

A bouncy rhyming tour of the world's oceans with bold, funny pictures. It sneaks in real geography without anyone noticing.
Good to know: Brilliant for the 'where do whales live?' stage.
See it on Amazon →
2
Best first fact book

Little Kids First Big Book of the Ocean - National Geographic Kids

30+ sea creatures with big, real photos. This is the book you grab when a question comes up that you can't answer.
Good to know: Great to dip into rather than read cover to cover.
See it on Amazon →
3
Best for 'but why?'

Who Makes an Ocean? - Sally Nicholls & Carolina Rabei

A gentle, beautiful story about how the ocean formed and how life began in it. Big ideas made small.
See it on Amazon →
4
Best before swimming lessons

Aqua Boy - Ken Wilson-Max

Aaron is scared of going under the water. BookTrust recommends it for ages 4-7, and it's perfect for nervous little swimmers.
See it on Amazon →
Mum tip: Pair a book with something real. Link the turtle page to a turtle you've actually seen, even at an aquarium, and the facts stick far better.
Verity's verdict
Verity hasn't tried this lot yet. Her honest verdict goes here once she has, good or bad.

Questions parents ask

Should I buy ocean stories or fact books?

Both. A story for bedtime and a fact book for the 'what's that?' moments. The fact book is the one you'll grab when you can't answer a question.

How do I get a 4 year old to sit for a fact book?

Don't read it cover to cover. Let them pick a page, follow their question, stop when they're done. Five minutes on octopuses counts.

Is there a good book for a child nervous about swimming?

Aqua Boy by Ken Wilson-Max. Aaron is scared of going under the water, and BookTrust recommends it for ages 4-7.