Pontus de la Gardie was a French nobleman who ended up fighting in Swedish service against the Russians, being given the Livonian governorate as a reward, who died in 1585 (the most common though looking into publications I've seen 1580 and 1583 also listed) by drowning. What's not clear is how this drowning took place.
An unsourced story I heard was that his boat was hit by a salute given in honour of a recent Swedish victory, after which he fell into the waters and drowned due to his armour. This is supported by the Russian Wikipedia, which notes this as one of many options for his death (I had to Google Translate this but the quality was very poor so I didn't want to copy it here). Also, I don't understand why the Russians cover this in more detail than the Swedes given Pontus was a Swedish landlord, governor, and favourite of the King.
Another source, Solovev's 'History of Russia' describes de la Gardie as having drowned while crossing the Narva River during some negotiations.
Is there any certainty as to how Pontus de la Gardie drowned? Which primary sources describe this event?
There are two answers at present with three different narratives, but none of these reference primary sources which would have covered this event.