Is a simple reaction between the copper in the bronze and the acetates and/or carbonates in the seawater.

Citric Acid (lemon juice - soak!!!) will remove/loosen the verdigris .... or you can buy 'stronger' concentrations of Citric Acid (Spotless Stainless™, etc.). A follow-up bronze wire rotary wheel should do the 'trick'. Verdigris is a surface 'oxidation' and rarely will lead to 'destruction' in the short term of a 'few years'.

To keep the green verdigris formation from bronze, I 'wire-wheel' it bright, then a few spray coats of clear urethane, lacquer, etc.