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Henry I, the 3rd norman King of England, died after eating a surfeit of lampreys after going on a hunting trip while ill. Apparently eating them was against the advice of his physician. Lampreys were pretty common fare in Early Medieval Britain but are pretty gross eel-like fish that still happily inhabit English rivers today. It is likely that they weren't properly cooked, and may have been contaminated with water from the river they were fished from. England rivers were littered with weirs and eel traps, and there were several Doomes demanding Weir clearance around the country in the 10th and 11th centuries (Law of Aethelred II) as they blocked the flow of the river, but rivers remained pretty clogged anyway and likely didn't dilute/run off the sewage and waste all that well!
John I 'Lackland' probably died of dysentery brought on by eating rotten peaches and drinking wine during a military campaign.
Edward IV died after catching a chill after a fishing trip. Some have hypothesized that he died of a stroke.
All of this information is on Wiki. Use this as a starting point to find better sources. I would wager, Henry I best matches your question.
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King Louis X of France died young in 1316 after a tennis match, I think. Possibly drinking something was connected with it.
King Edward VI of England died as a boy after a long and painful illness. It doesn't make medical sense today but I think I remember someone at the time blaming it on drinking water that was too cold.
I suggest that you might try researching the deaths of Louis X and Edward VI for anything about drinking water being the cause.
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Perhaps you are thinking of William the Conqueror.
The account of William's death from William of Malmesbury in his 12th century Gesta Regum Anglorum reads as follows in translation:
At last he set fire to the city of Mantes, where the church of St. Mary was burnt, together with a recluse who did not think it justifiable to quit her cell even under such an emergency; and the whole property of the citizens was destroyed. Exhilarated by this success, while furiously commanding his people to add fuel to the conflagration, he approached too near the flames, and contracted a disorder from the violence of the fire and the intenseness of the autumnal heat. Some say, that his horse leaping over a dangerous ditch, ruptured his rider, where his belly projected over the front of the saddle. Injured by this accident, he sounded a retreat, and returning to Rouen, as the malady increased he took to his bed. His physicians, when consulted, affirmed, from an inspection of his urine, that death was inevitable.