Upon researching 'Extreme Colonization,' you can colonize Aquatic, Frozen and Barren Worlds.  Frozen and Barren Worlds, however, incur a 50% penalty to productivity until you research Frozen World Mastery (which also provides a boost to Research and Influence on Frozen Worlds) or Barren World Mastery (which also provides a boost to Manufacturing and Military on Barren Worlds).

Aquatic Worlds never have a penalty to Productivity, however -- in fact as per the tooltip for an Aquatic planet on the left, you get a productivity boost.  You also always (after being able to colonize merely from having Extreme Colonization researched or using a Bazaar extreme colonization-capable special colony ships) get a boost to Morale and Tourism income.  I don't know whether Aquatic World Mastery provides an extra boost to Tourism, but I found no wealth boost from the tech (nothing listed on mouseover, unlike Morale which shows a 25% boost just from being an Aquatic world which is the same regardless of whether or not you have the Mastery tech).  

The descriptor for Aquatic worlds (at the bottom of the tooltip) might seem to imply you have some sort of available tile penalty prior to having the Mastery tech, but this does not seem to be the case; my Class-11 Aquatic planet had 11 total tiles to build on before and after getting the Aquatic Mastery tech.

My suggestion is to severely limit Aquatic worlds to just a couple of tiles until you research the tech (or are an Aquatic species) -- maybe a 50% reduction in how many tiles are available.  Aquatic Species should maybe have the Aquatic mastery tech disabled (I've never played Aquatic, maybe it is already?), as they already get the full bonus naturally and can thus research one of the other two Mastery techs.