There is no such thing as water proof, Eventually water will find a way in to anything if its continually exposed to water an example woud be dropping the controller into a bucket . The highest possible water resistance is IP67 this means the enclosure can resist water being sprayed at it from any direction for up to 30 minutes some Smart phones are now IP 67. Which is fine for getting caught in some rain, But Drop it into a bucket of water and its not going to end well.
As Spruce comments the Spring manufactured V11 are IP65 rated making them water resistant. IP 65 means the enclosure is resistant to water being sprayed at it form any direction for 15 minutes. The enclosures have a seal between faceplate and back box, so provided this is fitted correctly a little water being splashed or even some rain on it it should be fine. The Back Box itself has a sealed area protecting the PCB and the pre drilled mounting holes are outside of this seal so the controller can be mounted without effecting the IP water resistant seal.
There are a number of controllers used in the US on open trucks and of course widely on trolleys in the UK/EU. As the V11 is IP65 and provided the faceplate is fitted correct and the back box has not got extra holes drilled into it water splashing on to it and some rain should be Ok. The issue comes when water is left to pool on the top of the enclosure for long periods of time, this means that eventually it might seep into the enclosure.
My Advice would be if the top of the controller has some water on it wipe it off as you would your mobile phone, this prevents water pooling and building up over time. If you know the controller is going to be exposed for long periods to water for example very heavy rain over several hours using some electrical PVC insulation tape around the enclosure covering the seal between the faceplate and back box is very effective as would putting a small cover above the enclosure.
We very rarely see a controller come back with water damage to the PCB ( maybe a dozen over the last few years) even those fitted to trolleys. Where we do see them come back with water damage commonly the issue is holes have been drilled into the back box which are inside the seal. Water then over time runs down the mounting board and into the back box.
The controller is designed for use in a wet environment as you would expect. Fitted correctly and with a little care, even on a trolley the controllers are able to cope with a variety of conditions and give long service.