numbers STOCKH function for historical stock prices
The chart data for WTR on Yahoo Finance – which is indeed the data source – stops at February 24th, 2020.

So, there indeed seems to be no historical data for WTR after this date.

If you use "Feb 24 2020" as the date, Numbers shows the closing price perfectly fine.

Edit:

(a) Please note that STOCK("WTR") returns the price from January 30th, 2020.

(d) As a sidenote, STOCKH("APC.F",0,"29 Mar 2020") returns an error for me, which is correct as there is no trading in Frankfurt on weekends.

The function browser description recommends error handling by using IFERROR in front of the STOCKH function.