Basically, if you spin, then get out of the boat before it has stopped spinning, then re-enter the boat, it will cause it to start spinning where had left off at. Even if you leave it for a while it will still be spinning, and won;t continue as time is passing. It happens on ice and on water but is easier to test on ice. It is on the latest snapshot (16w41a) as well.
Related issues
duplicates
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Confirmed for 1.14.1