I need to make it so that when a player steps in a liquid, they have a regeneration effect on them for as long as they stay in the liquid. I've been wading through the source code of both vanilla minecraft and thermal expansion for half an hour trying to figure out how to get all the entities within a liquid block. so far I've had no luck

Maybe try this method?

 

public void onEntityWalking(World par1World, int par2, int par3, int par4, Entity par5Entity) 
{
       //Potion code
}