They could possibly not know that? Since the time line seems oldsy they might not understand properly how to freeze or boil water magic so that might be the reason they believe water is "useless". The MC comes from our world so he understands these concepts and how to properly do it but they dont,,,
yeah, I'm talking about the mc, I'm also used to mc who experiments with magic after finding out they're reborn in a world of magic so there's that.
Ahh, thats my fault! Thought you were talking about everyone in general LOL. And i agree, im also used to MC's like that but its sometimes good (and rare) to see an MC thats weak in the start and then becomes strong :)
but water is the most flexible element, if he could freeze it he could also boil it, there's a lot of possibilities to make use of water. Also water could be deadly if u put the pressure in one point, slicing even human bones (but this would be gory so)