Tides vary in height and are well predicted. Usually the low tide following a particularly high one will be especially low and it's useful to look for boat remains, in the mud, then. There's a cluster in the Tyne at Gateshead, between the Metro and Redheugh bridges. Oak timbers. We assume they're just a hundred or so years old - but could be any age really. Another thing to watch for at very low tides are the remnants of ancient bridges - bits of the Roman bridge over the Tyne are supposed to show then. It's unfortunate that the lowest tide of the day, on the Tyne, seems invariably at night.