Waterblasters can be very useful as part of the preparation process, helping to clean off dirt, grime and salt. If moss and mould is present treat that first with a product like Resene Moss & Mould Killer before waterblasting. Waterblasting will not kill moss and mould.
The water pressure of a waterblaster can vary dramatically and they can cause significant damage, such as broken windows and lights, if used inappropriately. Timber can be damaged by waterblasters and water can be forced up under weatherboards. Generally waterblasting is best for firmer surfaces such as concrete and steel. Ensure you use the appropriate pressure for the surface you are working on.