Depth Strider is supposed to increase walking speed underwater. Hence no changes when going up or down (no ground to walk on).
But while swimming around, you still get the bonus and can swim, way over the ocean/river ground, at max speed.

You can even start while standing still inside the river/ocean, get up a bit (you don't touch the floor anymore), then start running forward and it'll work.

I don't know if this is intended or not. Anyway, that's a good solution vs boats that aren't easy to deal with (talking about having them getting far away when touching them while sneaking off, and getting break when you just try to get them back to inventory items). But knowing if it is intended would be great