Underwater Hockey Turkey shared an interesting survey published by the CMAS a year and a half ago.
According to this underwater hockey is present in 27-29 countries, which I believe to be the same number for underwater rugby. The first second I started wondering how come they've never made a survey like that to measure underwater rugby? Previously you could find a post here about how many teams each sport has in how many countries. The numbers seemed quite accurate but it bothered me that I had no idea how many players do these teams cover as the numbers can vary a lot from 10 to 100.
A method I've used to estimate numbers is Facebook's Ad Manager. It shows the number of people with the specific interest, not the number of players/athletes, but it's something to start with.
Finswimming : 132.980. Finswimming has the highest numbers, but keep in mind that first of all finswimming is the only CMAS sports which gets real support from CMAS, and this way, goverments, also that numbers of participiant at competitions suggest that this includes swimmers who train with fins, scuba divers, UW games players, and so on, not only those who are interested purely in the discipline of finswimming.
Underwater hockey : 62.630. UWH is the older brother of UWR, it's fun and it's pretty well known and liked in France and in English-speaking countries. It has a huge adventage : it can be played in the regular swimming pools.
Underwater rugby : 44.760. UWR is one of the youngest teamsports nowdays, if we don't consider disputed sports like e-games and quidditch. Having this many people interested in it is quite impressive as it gets little to no support from CMAS or goverments and it's not the least viewer friendly sport without an underwater camera system. Also the low number of diving pools is an obstacle to overcome.
Water polo : 12.839.130. It's here for a comparism. If you follow water polo, you know that it's a great sport, but there aren't so many nations actively playing it. So how come the number is so high? It's FINA. They do a lot of marketing and water polo comes in a package with swimming to get people interested.
Another useful tool is Google Trends which shows and compares searches on different topics. Unfortunately Kinja has issues embeding the diagram so I can only give you a link to check my findings.
Taken a time period from 2010.01. till 2014.01. the average points are :
UWH - 58, finswimming - 19, UWR - 12.
Interesting fact that here UWH ranks much higher than finswimming. How is that possible? Keep in mind that finswimming is a rather obvious discipline and there is not much to explain about it. UW Games on the other hand get googled a lot more. Another reason why UWH ranks this high is cultural. It comes from English-speaking countries which are very active on the most common social networks. UWR on the other hand is popular in countries which all speak their own, different language so searches there are done in those languages. Other things to notice is that as interest builds up it usually peaks in a cover made by a larger medium, than it drops before it starts rising again. There also seems to be a correlation in the patterns. I think it's great proof that these sports go hand in hand and CMAS should be supporting them equaly the same way FINA handles its sports.
You can call me biased but on the long term I expect underwater rugby to have the largest rate of growth as it can provide the most adrenalin and intense action not only for the players but for viewers as well as long as the conditions are provided. I think it could live up to water polo as it is faster, more intense and more complex. To key to this is broadcasting and marketing which would require CMAS's support. Missing such opportunities like demonstrating the sport in Baku at the 1st European Games is a great mistake and just prolongs the stage when all efforts are done by the athletes. Recognising the needs of the sport should be the most essential for it's governing body, let's hope it will happen in the near future.