I had installed some gardening netting around the battery (fine mesh, stops insects), under the side panels, the concept being that spray and small water splash would get caught in the net and drain back, but still allows air to breathe, dry and not cause overheating. When I first looked under it when I got the bike, I could see that splash had reached up the sides of the battery enclosure, so I think that is possible without anything there. That is why I did it. It seems to work.
I can't see evidence of a tide mark so perhaps it was only just up to the pedals. I had to lift my feet, so was focused on riding like that than fall over.
Overall, the winter riding I have been doing, and now wading, I have inspected and my waterproofing seems to have worked so far. The box behind the headlight, around the wiring, seems to be doing what it is meant to do.
The 'goat leg'

I washed up the swing arm, and then washed it again after in a bit of distilled water, I do not trust tap water alone to wash bikes, especially this one, it seems to be largely made of mild steel tubes.
I will charge up and aim to take it on a fast ride tomorrow, make sure the motor is warmed up and rid of any moisture. There was no imbalance in the wheel when I spun it up, so I don't think there is any water in it.