Pieter will be happy to know that the first international tournament in underwater football (the 2016 European Championship) has been won by Belgium, beating England 5-1 in the final in Germany. 8 nations took part. They plan to hold it again next year, and have a World Cup in 2018. Austria, Switzerland, Germany, France, Italy and Spain also took part.
From what I can make out here (please can somebody who knows German correct me if necessary), it seems teams were allowed to use foreign players - even though there were only 3 players per team, not all the teams had enough players from their own country. Also, England seems to have had at least 1 woman in their team.
Belgium's victorious team was captained by Ünal Ölcücü.