Wetting and washing are two different things. I believe that anything that causes over manipulation during the washing/wetting period can do harm to your hair if you do it too many times per week.
Wetting to me is just getting under the shower for a few minutes one or more times a week and then holding in the moisture with a leave in afterwards. I do a lot of that but will do a co wash in the middle of the week when my hair feels a bit dry. And when I do a leave in, I realise that I need to wait for my hair to dry to damp so that it is not too much manipulation when I put my leave ins.
Washing which implies a shampoo and conditioner or a co-wash, can do some wear and tear on your hair if done too frequently under the shower head water for too long a period of time.
I personally think as long as you are gentle with your hair you can use water to help bring more moisture to your hair. One just has to keep in mind issues like frequency, how many minutes, water temperature, detangling method, detangling tools, how you manage your hair when it is sopping wet and your products (shampoo and conditioner).
If your hair feels sticky or dirty, I think a good water rinse can help. I have myself washed my hair with a light shampoo everyday when I am in West Africa and it is very hot. But I realize that that works much better when your hair is shorter.
Best,
Almond Eyes