I am using a small station, is that perhaps the reason that it isn't there.
Yes, it needs to be a large fire station with a water rescue expansion. The large expansion takes 24 hrs and costs 50,000 credits. Then the water rescue expansion takes 1 week and costs 100,000 credits. these can both be finished instantly using coins.
I would also suggest you train some people at the station in swift water rescue if you haven't already, as all water rescue calls need swift water rescue trained personnel.