Ok then as someone who achieved grade 9 in Eng Lang last year I will try and offer some advice....
Positives:
Very vivid sensory description with lots of good techniques used
Nice range of sentence forms
Nice circular structure at the end
Points of improvement:
Maybe practise using more advanced punctuation (semi colon, dashes etc) as you haven't quite used it correctly here, but this is a quick fix
Some more varied sentence starters - start with an adverb, a simile, a time expression - a useful technique Mr Salles on youtube recommends is to start each sentence with a different word. This will prevent you from using too much "The..." or "I...".
Try and think about your language choices a little bit more closely, For this question, you have to work really hard to sustain your tone throughout the piece and to make sure every language choice you make is effective. For example, "erupt like lava" seems a little cliche, could you think of a more original way to consider the threat of the waves? For example, "the waves invaded the town, devastating its civilians with relish" gives it almost a military feel. Then continue this theme throughout the writing (this is an extended metaphor). When you are considering every word that you write on the page closely to examine its effect, that is when you will get into the top bands. I hope this helps!