There are several agents which give creatures the ability to swim in C3/DS, with the injection of certain chemicals as a marker to tell the agent which creatures to give the ability to.
Swimmer Agent by Tomtschek
The first available swimming agent for C3 and DS, the swimmer agent is included with the Aquanorns and in the Aquagarden, made by Tomtschek. It requires Creatures to have the chemical Arnica in their bloodstream for the agent to take effect on them.
Swimming Agent by Nornagon
Swimming Agent V2 and V3 by Risen Angel
The Swimming Agent V3 was released as part of the CCSF 2016.