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For developing some fetishes, you have to meet initial requirements. For example, Blowjob requires Handjob of 50%; Bondage requires Blowjob of 10% and Teasing of 17% (see Walkthrough for more details.)
To start Piss Play (Watersport), you need Pussy Eating 50%. It can be gained not earlier than in the next version. So, Watersport as a fetish will appear in two updates.