Not sure about single player games, but most MMO's with swimming in (i remember when the swimming was first implemented in an mmo, everyone went nuts over it!) usually use holding the space to take you and keep you on the surface, if you take you finger off space you slowly sink to the bottom, if you hold space you slowly rise, and so on.
i7 7700,k NV1070 8GB, 16GB 3200mhz memory, 1x 2TB Hybrid, Win10.
i5 4760k, NV960 2GB, 16GB 2333mhz memory, 1x 2TB Hybrid, Win10.
i3, Intel 4000 series graphics, 6GB memory, 1x 500gb HDD, Win8.1.