Blisters, re the swim. This year the current was good, and apart from one section, I dodn't notice much difference between up and downstream. I did do a river swim the other side of Henley (Shiplake) in the summer, and the current was much stronger : it just took forever to get to the turnaround point, then you were back to the finish in no time.
there are several river swims and shorter distance tri's held over the summer locally, so there will be somewhere where you can try it out if you want to.
I don't think the full distance sold out this year - or if it did it was a last minute thing - so don't worry about entering the race as soon as it opens.