Depends totally on what you are aiming for.
Fluid simulations may work well if you want characters not only drown in semen but have them interact physically with it as well. Otherwise these may be too heavy for what you are trying to achieve.
Most likely you will want to look at how to dynamically paint surfaces with cum. Best example here would be Splatoon, just with jizz instead of paint, eh?
Here's a video explaining the basics behind such a system:
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