We skied over a decade on a river with quite the current before moving to Florida.
There are actually two effects to the skiing.
First it’s the actual speed through the water.
Boat speed is set by gps, so fixed or over ground as you will.
Let’s make it easy and say, you have a current of 2mph. Boat travels at exactly 34mph (over ground).
Skiing downstream, the skier will ski @ 34mph above ground, but only 32mph through the water.
The opposite is true when skiing upstream. Boat - again pulls @ 34mph above ground - skier though will get pulled through the water @ 36mph. Feels quite weird and very different from the last pass!
Secondly, due to the current flowing in one direction through the course, the distance between the buoys feels totally different going upstream vs going downstream. Not side to side of course, but almost like the whole course is shorter (clenched?) downstream vs longer (stretched) upstream. That greatly effects your timing!
I always found it quite challenging and always skied my best going against the current (upstream).