The Rhine in Germany as many rivers here gets the usual spring flood in May or June or both. The water is very high for a few days and then pretty high for many weeks. There are quite a few forests and hundreds of thousands of trees alongside the river which are under water for weeks. Many are almost swamp-trees like willows, but many others are just ordinary trees which will grow in the forests on the hill too. According to conventional bonsai wisdom they all MUST die because they are standing in water for weeks. Why don't they die? Well, the water is full of oxygen and the banks of the river are of pure sand and grit. The tree standing in this sand will get a constant flow of fresh water UNDER the soil surface right where the roots are constantly. All this water is full of oxygen. while there are lots and lots of organic matter which decompose and use up oxygen they don't do harm because the water is moving.
Can we learn a lesson here for bonsai culture? it is NOT about water in the soil or substrate. it is all about oxygen getting to the root tips or not.