James Mcavloy was the best thing about Split, but I agree the film itself was Mediocre. I haven't seen Lady in the Water but it's now added to my watchlist (a watchlist I now see is at 2,134 title!)
The 90's and 80's brought us some of the best slashers, with over the top deaths and gratuitous nudity that sadly we tend not to see as much today.
Is it perhaps one of the Friday the 13th's?