From the dark abyssal depth to the shallow blue lagoon of holiday bliss the sea is a wealth of worlds waiting to be explored. Write down all the different characteristics you can think off that would represent the sea, create a person or persona to represent each of these aspects and write a stanza for each.

Maybe have them interact in further stanzas.

Initially do not worry too much about the number of lines in each stanza or even if they are rhyming or not. You will probably find that an almost structure will appear by itself which you can letter edit the poem to fit better.