I think that one of the most intense metaphors in Ondaatje's poem would be the sea waters which is introduced in lines 25- 29. Ondaatje's metaphor of the voyage on sea waters is an important one in the poem:
One day I'll come swimming
beside your ship or someone will
and if you hear the siren
listen to it. For if you close your ears
only nothing happens. You will never change.
The metaphor of sea waters is important for a couple of reasons. The first is that it communicates the difficulty of parenting. The entire poem is concerned with how this father can effectively express his love for his daughter and how a loving parent can care for "a sad daughter." It is filled with turbulence, joy, moments of despair being matched with moments of elation. The idea of the relationship between them as a ship set amidst challenging waters is an effective metaphor to describe the difficulty of being a parent.
I think that another reason why the metaphor of the ship set on the waters is effective is because it speaks to the condition of being in love with another. Whether it is love of a child or love of a spouse, Ondaatje shows love to be an experience set adrift amidst a sea of insecurity and doubt. The ship metaphor becomes extremely effective in this light. When one loves another, there is always a feeling of being set amidst a setting where someone "swimming" to help us is always craved. Even the hearing of "sirens" can be helpful when so uncertain of what to do, how to love, or how to help someone who is distant and does not seem to respond to help. This becomes an effective metaphor because it speaks to the lack of answers and lack of focused direction that one experiences when in love. When trying to reach a "sad daughter," one is in tough sea waters.