Where is the key to the first door

Hint 1 of 7:

First, you must read the writing on the door.

Hint 2 of 7:

A mermaid can help your quest, if she wants to.

Hint 3 of 7:

Give her the bouquet of flowers to get an underwater taxi.

Hint 4 of 7:

Doesn't Neptune look like he's missing something?

Hint 5 of 7:

Have you searched along the beach?