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
The Swimming Agent by nornagon gives your Norns the ability to swim in water. Originally intended for use with Aquatilis Caverna, and is only usable in Docking Station, not C3 standalone.
Can be found at Creatures Caves.
Swimming Agent V2 and V3 by Grendel Man
The Swimming Agent V2 by Grendel Man gives creatures the ability to retreat when swimming. Creatures must be injected with chemical 63 in order to use this agent.
Available at The Realm.
The Swimming Agent V3 was released by Grendel Man as part of the CCSF 2016.