Yes Showjumping is probably the ultimate posh due to the cost of the horses you need as basic equipment
If Eton school don't produce generations of brilliant rowers, with that facility on their doorstep, we would like to know why. Hope that other local schools can also get to use it. In NZ rowing is non-posh. It helps that they have a number of great rivers and sheltered bays to do it in. In a smallish town like Wanganui there are 2 rowing clubs.
Here is it mainly done by posh schools and universities with access to the few decent rivers. My DH did rowing in Bangor Uni - they used the choppy Menai Straits love 'em.
You can see if you walk around Stowe (Branson's old public school - grounds now in NT care) the facilities the school students have, including their own golf course and shooting range.
I have never met a non-posh sailing person jeni. In my (ex) school the kids did get a chance to do a bit of sailing. But their parents are no more likely to get them to regular sessions in the local lake than they are to book them in for flying lessons. Apart from anything else lessons in sailing, riding etc do not come free. Our school, although sporty, and although school has good facilities, never win anything in sports, because the kids do not have parents who take them to things outside schools. (Apart from a very few who do things like judo, boxing and football outside school) And, it has to be said, they are not as well nourished as kids from more middle class schools. The PE teachers see the difference in size! And then there is the obsession with soccer... Where does that get us... Mo Farah had a lucky escape when his PE teacher steered him away from soccer and into running.