Yes your box braids and or twists can get wet, especially if i installed them. I know not everyone has access to me but even if u got some sad diy boxbraids and twist they too can get wet.
So either way go for it I say "jump in, the waters fine". If u swim in a pool remember the water is chlorinated and therefore I recommend rinsing hair thoroughly after, the same goes for salt water. I do not recommend washing hair every day and even when and if you do wash your hair there is a special way to do it. If you do not yet know how to wash your box braids and twists don't worry there are resources on this site.
The best time to get your braids or twists wet is within the 1st month when your hair hasn't begun to sprout out much and it's still safely encased in the braid. Everyone's hair texture is different but most of us who wear box braids have the type of hair that expands when wet so keep that in mind.
After hair is wet and dried it might lose its luster though that can be replenished, "nobody got time for that". Once your braids or twist have gotten a bit older getting them wet will not be an issue because they will have already be broken in and have a more natural look and feel. Still at the end of the day it's all up to you.
I recommend giving your braids or twists a good squeeze with a towel , adding your favorite oil or oils and air drying for best results.