Diesel engines do not respond well to water in the fuel, as it impairs combustion. Storage tanks should be regularly drained or cleaned to remove water buildup and preventative measures such as breathers are used to reduce ingress.
It also causes the perfect conditions for life, meaning you are at risk of fuel bugs that get trapped in the wet filter, grow and block the filter.
The earliest sign may be loss of power as fuel is no longer able to be delivered to injectors because of blocked filters.
Fuel analysis identifies fuel bugs early before the failure. So why not catch it early with a laboratory fuel analysis.