Kamasutra 2.6.34

2.6.34

jale ca saṃviṣṭopaviṣṭasthitātmakāṃś citrān yogān upalakṣayet/ tathā sukaratvād iti suvarṇanābhaḥ//

Thus, says Suvarnanabha, these different ways of lying down, sitting, and standing should be practiced in water, because it is easy to do so therein. But Vatsyayana is of opinion that congress in water is improper, because it is prohibited by the religious law.