I just learned the word 泥水 and guessed it to be read as どろみず. When I checked what it was, WaniKani listed でいすい first, then どろみず. I had a look online, and according to this forum どろみず is far more common.
Although I can’t remember any good ones off the top of my head, I know that there are a bunch of other words that can be read either as all on’yomi or all kun’yomi. With these words, do Japanese people tend to read it all kun’yomi, like どろみず apparently is? Is it okay for me to default to that?
It would depend on the individual word. Tons of words that have common onyomi readings have obscure kunyomi readings. And vice versa. Sometimes both are common words. You can’t just check if one is possible and always default to it.
In some cases, it would just change the meaning. Like 白鳥 read as はくちょう is a swan, but 白鳥 read as しらとり is “white bird,” which could be a swan, but it can indicate other birds as well, like seagulls.
Judging by the responses in HiNative, どろみず seems to be far more used in everyday life and でいすい seems to be used more in television and newspapers. I remember (though I might be wrong) reading that on’yomi words tend to be more formal than kun’yomi counterparts.
It makes sense when you think about it. Kun’yomi readings come from the spoken language japanese people spoke before writing was introduced to them, while on’yomi comes from the study of chinese writings.
I would expect commoners and farmers to prefer the kun’yomi reading while the nobles would favour using the on’yomi.