http://wiki.openstreetmap.org/wiki/Proposed_features/barriers ?
http://wiki.openstreetmap.org/wiki/Harbour ?
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RP: i would suggest creating waterway=floodgate and documenting it on the wiki, or extending the barriers key. wither would be valid, there is some crossover here
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HB: waterway=floodgate seems like the better solution to me. I read 'barrier' more as a barrier to people, not streams.
see also semi-related http://wiki.openstreetmap.org/wiki/Tag:waterway%3Dlock_gate
http://wiki.openstreetmap.org/wiki/Tag:pipeline%3Dvalve