A simple concept. A large magnet on a long piece of rope thrown into the murky depths of any body of water you have access to. The reality though of course, like actual fishing, is a lot harder than simple.
My magnet is 15cm in diameter (thanks Amazon) with a large eyelet for easy rope tying. I have 20 metres of rope, which is doubled up on shallower water. Wrapping the rope end around my non-throwing arm ensures I don't send the lot flying into the abyss.
A pair of rubber gloves and a towel complete the ensemble, as believe me things get mucky, not to mention smelly. No major finds yet but every retrieval is like Christmas; you never know what you're going to get.
Ordnance Survey and Bing maps are a great source to find rivers, canals etc, crossed by footpaths or bridges on roads. Note: If you chance your arm, or magnet, on private waters, don't expect a warm welcome with tea and biscuits.