Put the strawberries, water and sugar in a saucepan and boil gently, for about 5 minutes.
When the strawberries are soft, mash with a fork or a potato masher. If you don't want strawberry bits/lumps in your mousse, process in a food blender. (I used a masher as I like lumps of strawberries in my mousse)
While the strawberry mixture is still hot, pour in the marshmallows and stir until dissolved.
Pour into a big bowl to cool.
Crush the biscuits, pour in the liquor if using. Divide the crumbs between 4 ramekins.
In a separate bowl, whip the cream until it holds its shape, then fold into the cooled strawberry mixture.
Pour the mixture over the biscuits and refrigerate for 2 hours, or until set.
Before serving, put some halted strawberries on top to decorate.