Setting: Sweden during the Middle Ages, in a lake or deep river.
Search terms: I've tried everything from "water as a lubricant" to "sex under water", but have just found a whole bunch of scuba porn sites. I've also scoured various sites about gay sex, such as "Minotaur's Sex Tips For Slash Writers".
ETA: You guys are too awesome for words; within minutes I got the answers I needed. Thanks a million!