Place the crab into a mixing bowl and squeeze over the juice from half a lime. Add the red onion, watermelon, cucumber, garlic, scotch bonnets and tomato. Add salt and pepper and stir well.
Next make the green seasoning vinaigrette by adding the green seasoning, rum, sugar and the juice from the remaining half a lime to a small bowl. Season with a little salt and pepper. Whisk the mixture whilst slowly adding the olive oil.
Add the chopped herbs to the crab mixture and dress the crab with a few tablespoons of vinaigrette. Mix well so that all the crab is coasted and moist.
Fill the centre of each avocado half with the crab mixture. Drizzle and little more vinaigrette over each one and garnish with a sprig of parsley.