Enabled water, left bump map disabled... Ended up white screening out in about 1.5 hours.
Disabled water, re-enabled bump, and played a few sessions (some longer than 2 hours) without crashing.
Appears to be the water. Will see if I disconnect in scenario frame 2 tomorrow this way (the easiest long-session test).
EDIT: oddly enough, if it IS the water detail box, this has occurred very far inland with no water around anywhere, in the past. Very strange.