Outer Limits 4x4 Board

I noticed on the way to mackay today the volt meter dropped to halfway. On the multimeter was outputting 13.5V but should be around 14 I think. I know a week earlier went thru (but not stuck) in some muddy water. I took it to car lovers tonight here in mackay and hosed the alternator out and seems ok now pumping out 14.2V. If I recall it did this before and it fixed it but what exactly? Would inside would have been affected by the dirty water to then work with a clean out it would be handy to know. Regulator maybe?

Any help appreciated