From FINA WATER POLO RULES:
WP 20 ORDINARY FOULS
WP 20.1 It shall be an ordinary foul to commit any of the following offences (WP 20.2 to WP 20.17), which shall be punished by awarding of a free throw to the opposing team.
WP 20.4 To hold or push off from the goal posts or their fixtures, to hold or push off from the sides or ends of the pool ...
Here's a start, three ideas:
Top-down: which countries play waterpolo (check WC results/qualifiers?). Then guess the % of inhabitants playing WP.
Bottom-up: find the amount of pools where they play WP or amount of teams and then combine that to a total number.
Alternatively: look up the stats for all the (biggest) countries individually: for example ...