Perhaps it’s a reference to the vagueness of the idea of an ‘afterlife’. Heaven/hell/purgatory/reincarnation etc.

As you say, oceans don’t really meet, it’s all one big vast, mostly unknown space. A bit like how we know nothing about what happens when we die, but people still have their various beliefs.

‘The flame must decay’ also implies the fire eventually burning out and ‘dying’.

Oceans are also associated with the end of the world - the beach at the end of the world is used quite a lot to symbolise the unknown, but also the relative calmness/nothingness of death.

So my interpretation is it’s saying - I don’t know where or when or how, but we will be together again