This hymn was written more than a century ago, back when more people were aware of how Early Modern English arranged its conjugations. But in the second verse, there appears to be an inconsistency between two otherwise identical instances of "watch" — tense, person, and so on are all the same.

>O the deep, deep love of Jesus,  
Spread His praise from shore to shore!  
How He loveth, ever loveth,  
Changeth never, nevermore;  
How He **watches** o’er His loved ones,  
Died to call them all His own;  
How for them He intercedeth,  
**Watcheth** o’er them from the throne.

Mostly this is using -eth, including the last occurrence, as one would expect. But the one in the middle is really strange. It's not an accident either; [the version with -eth corrected](https://www.google.com/search?q=%22o+the+deep+deep+love+of+Jesus%22+%22how+He+watcheth%22&start=10) is an order of magnitude less common than [the one I quoted](https://www.google.com/search?q=%22o+the+deep+deep+love+of+Jesus%22+%22how+He+watches%22&start=130). Is there some hidden meaning to this difference?