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.
[img]belidos3d.swgrp.co.uk/wp-content/uploads/2016/03/blogo.png[/img]
Primary Desktop:
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