"Welters" is a verb that can refer to a variety of different actions, including rolling around, wallowing, or floundering. As such, there are a number of different synonyms that can be used to describe similar actions. Some possible alternatives might include terms like "flail," "struggle," "squirm," "thrash," or "toss." Other options might include "flop," "flounder," "lurch," or "plunge." Ultimately, the specific choice of synonym will likely depend on the context in which the verb is being used, as well as the tone or connotation that the writer is hoping to convey.