Your barn manager is probably being overcautious.
One of the other possible causes of founder is a restriction of blood flow to the hoof , so he may be thinking that to allow the horses into the cold water may be a possible danger due to the arteries & veins contracting as a result.
This is a an area of research that is still under investigation, but I think they are talking about just submerging the feet in ice cold water over prolonged periods of time.
I don't think that exposing the whole animal can cause a problem - if it did none of them would make it through winter without having problems.
What I would do in your case is ---- enjoy yourself !