I haven't seen official word about it, but...
https://steamcommunity.com/sharedfiles/filedetails/?id=2432715512
water is literally tranparent and the bottom of ocean is empty
I assume that devs don't plan to add diving/stuff at all

If they did plan such stuff... there would be at least something to handle water screen when you are underwater
Not to mention mobs are made in a way that either water pushes them up to surface or sinks them down to the bottom, without mods you can't get mobs to swim up/down to attack you, you are literally immune when inside of water (there is no oxygen either)