It's not such a dumb question actually. Any river will have a certain amount of fresh in it, although as far as this question is concerned I've not yet checked but I am sure others have assured the questioner that zinc will be fine on the Hamble.
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HiYa Martin, I moored on the Hamble for years. I can tell you quite categorically that fresh water is found about a half mile above Bursledon Bridge at Low Water. From then on up even with the tide in it gets progessively less saline.