No correction, the lyrics are fine. They mean...(Google translation + my editing)
1 The sea that we see dancing along the clear gulfs has reflections of silver, the sea, changing reflections under the rain.
2 The sea in the summer sky merges its white sheep with the angels so pure, the sea, endless shepherdess of azure.
3 See near the lagoons* these big wet reeds. See these white birds and these rusty houses, the sea lulled them along the clear gulfs and with a love song, the sea, has lulled my heart for life.

*Note that what we call "étang" near the Mediterranean are not ponds but lagoons. Trénet was from Narbonne and if you put Narbonne into Google maps you'll see that there are many lagoons along the coast and they're all called "Étang so and so".

About "Coucou", the request is almost 20 years old but here they are anyway

Coucou, les rosiers fleurissent
Coucou, les rameaux verdissent
Coucou, voici le printemps

Coucou, le beau soleil brille
Coucou, et les yeux des filles
Coucou, en font tout autant
Que faites-vous, que faites-vous
Encore à sommeiller

Éveillez-vous, éveillez-vous,
Le monde est transformé
Coucou, ouvrez-moi bien vite
Coucou, mon cœur vous invite
Coucou, il faut nous aimer

Cadillac Cadillac
Coucou bonjour mon amour
Cadilla dilla dilla Cadillac Cadillac
Coucou veut dire bonjour...

... which means... (ditto as above)

Hello, the roses are blooming
Hello, the twigs/branches are turning green
Hello, here is spring.

Hello, the beautiful sun is shining
Hello, and the girls' eyes
Hello, do the same.

What are you doing, what are you doing
[You're] still sleeping.
Awake, awake,
The world is transformed
Hello, open me up quickly
Hello, my heart invites you
Hello, we must love each other

Cadillac Cadillac
Hello hello my love
Cadilla dilla dilla Cadillac Cadillac
"Coucou" means hello

Now... "coucou" literally means "cuckoo" -the bird. French cuckoos sing "coucou!" and the word is used to play peekaboo with babies. It's also used to say hi/hey there to family and close friends, either when you meet them or when you phone or email them.
Note that "coucou" never ever means crazy and that's why the meaning of the movie title "One Flew Over the Cuckoo's Nest" translated as "Vol au-dessus d'un nid de coucou" must have been lost in translation for most of the French.