It's unclear. First of all, it should be 'on the seashore', although he does say 'in'. He also has a strong Jamaican accent I have difficulties following at times, and speaks in what I'd guess is the Jamaican patois at the beginning.
In the sentence before "walking in the seashore with my future in the sand" he says "walking in the seaside with my (lightning?) in my hand". I'm not sure what he actually says.
I'd guess that you're correct in that he's talking about the mutability of his future. Or perhaps it's a reference to divination, like reading tea leaves only with sea foam?

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