Vintage Picture Book (from 50s, 60’s, or 70s), not very thick, stapled, maybe 10 – 12 pages, not a lot of printed text. Watercolor, realistic illustrations

What I remember about the story: A boy spends a lot of time snorkeling, he sees all kinds of fish and turtles, and he finds a shipwreck and a shark shows up (scary) but he escapes and finds a treasure.

I remember the following illustrations:

1. Sea stars on a sandy ocean floor, the sunlight is shining through the water onto the ocean floor
2. A sea turtle swims and again the sunlight coming through the water
3. A two page illustration showing the boy on the left top looking down on the shipwreck, again the light plays through the water

The illustrations were realistic and what stood out was the light portrayed underwater.