Wide angle lens is certainly plus for UW, but it can be rather expensive.
Perhaps you could try with your current setup for few months, see if you are happy with the photos and then decide.
As far as filters are concerned, I try to get maximum amount of information into the raw photo (no filtering) and then postprocess it in Photoshop if needed (filters, removal of backscatter, slight recovery of lost colouration).
Actually, that is what I would like to do, but I never find time to mess with it so I leave photos in raw format.
