They seem to be sometimes used interchangeably. Of course, 'marine' refers only to saltwater, not freshwater, so in that sense, it is more limited in scope than 'water sports', which would include such sports as whitewater rafting. In my own mind, I envision 'water sports' as being simpler, more familiar, or more traditional ones like swimming, volleyball, and waterskiing, while marine sports I think of as more sophisticated — scuba diving, sea kayaking, jet skiing, etc.
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