I just purchased one of the grey looking Lifetime deck boxes, they had them at Lowes.
Work well for the price!
http://www.lowes.com/pd/LIFETIME-PRO...ck-Box/3169925
A few of us on our dock are using these. It's not officially a 'dock box', but for $148 it looks good, is fairly dry (no water getting in mine) and can be locked to keep the honest people out.