Strong adhesives underwater play an important role in diverse fields, including biological tissues and biomedical applications, but have proven to be challenging. Marine organisms affixing themselves to rock under seawater with the help of 3,4-dihydroxyphenylalanine (DOPA), evoke an intriguing inspiration on wet adhesives. However, current underwater adhesives are not easily applicable in large part due to either cohesive or adhesive failure. We report a strong adhesive, with enhanced cohesion by designed covalent bonding. Strong coacervate adhesives were formed underwater by simply injecting a mixture of two DOPA functionalized polyelectrolytes, DOPA grafted polyacrylic acid and DOPA grafted polyallylamine. Oxidative crosslinking was designed by inducing controlled amounts of sodium periodate. Adhesion occured within minutes with cohesion obtained through a combination of electrostatic interactions, hydrogen bonding and covalent bonding. The adhesive abilities of both wet and dry adhesives were tested on glass substrates by lap shear test. This strong adhesive can be easily appliedand may have potential applications in many areas, including tissue adhesives, wound dressing, etc.