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There are languages around you. See them. Trace their letters and sound them out. Then, there are the underwater languages. See them. Trace their letters, sound out the colours you can't see. Taste their letters. Is the message salty or sweet? Feel their letters. What type of seaweed was used? Slim? Old? Young? Words are more than letters; letters are more than what one sees.

— overheard from a teacher and student walking the beach

There are several ways to read languages and underwater beings use almost all of them. While tree-based beings such as Dryads will use scents and flowery perfumes to add their thoughts in communications, sea-beings will add letters in colors not seen by many land dwellers, in tastes that can be "drank" in nearby ocean water, and even with the type of seaweed.

It will also depend on who is sending the message and their relationship to the person receiving the message. 


Overall, while land beings only think about this as being a language used by all, ocean spies (always sending back information to their keepers) use this as a coded/secret language. It's a balancing act, because any ocean-creature can see that the message is not what it seems, however it would break the bond of sea and the privacy that creatures in oceans worldwide grow up knowing. 
While there are specifics - shark-based beings use a punctuating form of letters, with their teeth size and shape determining the meaning, the language mostly used for spying is from the octopi-beings.

• personal ink (or ink from another) is mixed with colors that can't always be seen on land
• the ink is chemically altered, taken from the sacs that hold them and mixed into hardened bubbles. 
• when needed, a bubble is pierced with a quill
  

Layers matter as well. The 1st layer of the message is color, the 2nd layer of the message is texture, and the third layer is taste. What happens is that each layer can have additional words to make 1 complete messager OR each layer adds so that the message changes the more that is read.

For example, adding a 1st layer word of fire + a 2nd layer word of city + a 3rd layer word of politics shows a single message about the political situation of a city.  Or, adding a 1st layer sentence of how the city has been in the recent past + a 2nd layer sentence of current weaknesses + a 3rd layer of what should be done in the future shows the message as a whole.
 
There's also the flower language of the ocean, with how seaweed can be given and received to show which relationship a being cherishes or what relationship they would like to be in. In this case only the color and texture are used for meanings - the taste of seaweed is not needed. 
Senses in Messages
Taste: 

• salty - best/great 
• sweet - suspicious
• sour - bad  
• bitter - okay (not good/not bad)
• savory - good

Texture:

• firm
• bulbous
• young/old
• polyup
• smooth
• rough
• clean cut / ragged

Examples
Ragged + bitter = a little less than good, dipping into bad Wordwise, the message could mean bad for the other person and a good time to take advantage.