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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.
Meet the Oceans - Caryl Hart & Bethan Woollvin
Little Kids First Big Book of the Ocean - National Geographic Kids
Who Makes an Ocean? - Sally Nicholls & Carolina Rabei
Aqua Boy - Ken Wilson-Max
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.