Robb was correct! This school of fish drops 100% Peacbloom. Shool of fish last for quite long, but disapears after some time. Don't know respawn rate though.

Schools of Fish will always look the same, whether they are in a lake, a river, or the sea, namely a flashing circle in the water with fish darting through the circle.
When hovering the mouse over the school of fish, a description will pop up describing the type of fish congregating in it.

Your fishing bobber needs to land in the circle or at least very close to it, then you will either receive the respective fish or the occasional other item such as Stranglekelp, Ramsey Rum, or Watertight Trunks.

When the School of Fish has been fished empty, it will disappear. One or two catches before the school is emptied out, the circle will shrink a little and begin to flash somewhat faster.

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By the way, circles of Floating Wreckage work exactly the same way as do Schools of Fish. But here, you will receive mostly non-fish items such as waterlogged cases or such.