This paper aims to motivate more research works on the design of zero-bit watermarking schemes by showing an upper bound of the performances that known solutions failed to reach. To this end, an upper bound of error exponent characteristic is derived by translating Costa's rationale to zero-bit watermarking with side information. Three schemes are then considered: the dual-cone detection region originally proposed by Cox et al. and improved in Merhav et al. papers, ISS (Improved Spread Spectrum), and ZATT (Zero Attraction). It turns out that in certain conditions the latter performs better than the first one, which questions the optimality claimed Merhav et al. Nevertheless, the main conclusion is that these schemes are in general far away from the upper bound in the region of practical interest.