Slice the pineapple in half lengthwise, leaving the greens attached. Use a paring knife to cut away the fruit and core from both halves of the pineapple, leaving a thin border of fruit intact to make 2 pineapple boats. Cut the fruit into chunks and set aside. Place the pineapple boats in the freezer until you are ready to serve.
Peel the banana and cut into chunks. Place the pineapple and banana chunks on a parchment-lined baking sheet or dish and freeze until frozen, about 4 hours.
In the bowl of a food processor, combine the frozen fruit and cream of coconut. Pulse until the mixture forms a thick paste, about 3 minutes. Gradually drizzle in 1/2 cup water, pulsing, until the consistency of soft serve is reached.
Scoop the soft serve into the frozen pineapple halves and garnish each with maraschino cherries and a small bottle of rum if desired. Serve immediately.
You can substitute 4 cups frozen pineapple chunks for the whole pineapple. Spoon the frozen treat into 2 serving bowls and garnish each with maraschino cherries and a mini rum bottle if desired.