Aquatic mobs seem to have difficulty pathfinding. They get stuck on sides of blocks that usually can be swam up easily. Many also swim to the bottom and can't swim back up. If there is a hole in the ground they seem to prefer it and get stuck there as well. This doesn't apply to Drowned.
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When you make a 1x1 hole in the bottom of a river it gets filled with down-flowing water, and that will pull mobs and items downward.
There is clearly down-flowing water in this screenshot:
These screenshots also seem to show down-flowing water (based on the lack of a bubble column above the soul sand, and the items resting on the bottom:
[media]
The other screenshots also show that you've been modifying the water by placing, and perhaps removing, blocks. In doing so, you may created some areas of down-flowing water that are large enough and oriented in such a way that the source water sources around them cannot turn them back into source water.
Does that explain your issues?
If not, do these reports explain what you are seeing: MCPE-37005 and MCPE-131091?
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This happens in both Creative and Survival. I was goofing around in Creative here but the aquatic mobs will get stuck in rivers too. Oceans don't have this problem as often. I'm unsure how Guardians are affected but Elder Guardians are kinda wonk. Don't know if that's intentional or not.