Driving through town when it started to bucket down. Quickly had half a meter of water in the low spot.

Drove through a deep spot, water past the bottom door seals (no water got in).

Short time later noticed the water in fule light was on. After a day of normal driving (and clear, hot weather) it went out and hasn't come back on.

Any idea what could be broke or cracked to allow water in like this?

There would be a sensor underneath, if its only come on once I wouldn't worry about it perhaps the contacts got a bit wet.

That's what I've been doing!

Vehicle was in storage for 2 years prior to purchase. Loads of perished parts had to be replaced. Think I'll get a can of liquid rubber and coat the outer connector housing.