The RCE states the following :
"Died on(sic) the river Clyde 500 yards from the Erskine ferry (Location was just below the modern day Erskine bridge). "
"Deceased was subject to fits and probably fell into river while attacked by a fit"
RCE is speculative, no witness no real evidence!
It would suggest that Agnes suffered from Epilepsy.
I have no other records with regard to any other members in the tree who had epilepsy.