Put half of the wakame into a bowl of cold water. Leave to rehydrate for about 5 minutes. Put the rehydrated wakame and 3 tbsp of its soaking water into the blender. Blend to a rough paste. Remove from the blender and put to one side.
Sift the flour and baking powder and put them into a clean blender with the other half of the wakame - which is still dry. Blend until the wakame has broken down into breadcrumb sized pieces. Now tip the flour mix into a bowl.
Make a well in the centre of the flour mix and add the oil and the wakame paste, mix with your hand to a smooth dough, which is soft but not sticky - add a little more water if necessary.
Leave the pastry to rest for 15-20 minutes covered in cling film.
Preheat the oven to 160 degrees C/Gas 3. Lightly flour a clean work surface and roll the pastry out as thin as you can get it - 2-3mm thick is ideal. Cut the pastry into even squares - 5cm approx - and carefully transfer to a baking tray lined with parchment paper. Press three fork marks into each, brush with water and sprinkle on a little sea salt. ‘A little' is the key word here. If you add too much they will be very salty.
Bake the crackers for 12-15 minutes or until they have become hard and very slightly golden. Remove from the oven. Cool on a wire rack and then serve.
Storage: they keep for around a week in an air tight container.