Underwater wireless sensor network is a novel type of underwater networked system. Due to the characteristics of UWSN and underwater channel, UWSN are vulnerable to malicious attacks. The existing security solutions proposed for WSN cannot be used directly in UWSN. Moreover, most of these solutions are layer wise. In this paper, we studied the threat, attack, and security issues of UWSN. Subsequently, we argue that layered security schemes cannot protect UWSN against blended attacks. Additionally, we presented a novel preliminary conceive, which is a cross-layer, adaptive, selective security scheme.