In reply to j0ntyg:
> (In reply to balmybaldwin)
>
> >> Thanks for that. Would I be right in assuming that a thin tyre has the advantage of being compressed down through the water on the road surface,
> to the road surface itself? There will be an incisive tyre plus the weight of the rider.
It's that sort of thing, yes. When a car aquaplanes, it's because the pressure the car's weight is exerting on the contact patch is low enough that surface tension can momentarily support the car's weight. Deep tread helps displace the water before this can happen, provided speeds are low enough.
On a bike, the contact patch is much smaller and is only two wheels, leading to a much higher contact patch pressure. Hence, it's impossible for a pedal bike to aquaplane.
Wet roads are of course more slippy, but getting more contact rubber on the floor is more important than having measures (tread) against something that can't happen (aquaplaning). Dropping the tyre pressures slightly gives you a slight increase in contact patch, hence increasing your limit of friction on corners and for power transmission.
There's a really good Sheldon Brown article about all this somewhere.