Watters was commenting on former special USA envoy for Ukraine Kurt Volker's testimony before Congress, which took place Tuesday, with his co-hosts.
Ms Williams told the committee that Mr Trump's call with Mr Zelenskiy was unusual because "it involved discussion of what appeared to be a domestic political matter".
The White House National Security Council (NSC)'s top Ukraine expert testified that he had been concerned by the president's demands to investigate Mr Biden.
On Tuesday, he said Gordon Sondland, the USA ambassador to the European Union, made a "generic" comment about investigations that everyone regarded as inappropriate.
Depending on what happens in the next few weeks, Mr Trump could end up facing impeachment.
These were allegedly used to bribe the Ukrainian leader to either find or invent dirt on Hunter Biden, the son of the former vice president and current Democratic presidential candidate Joe Biden.
At the inquiry's core is a July 25 telephone call in which Trump pressed Zelensky to investigate Biden and his son, a member of the board of Burisma, a Ukrainian energy firm, and a discredited theory that it was Ukraine - not Russian Federation as determined by US intelligence agencies - that interfered in the 2016 USA election.
Sitting next to Vindman, Jennifer Williams, a foreign policy advisor to Vice President Mike Pence, said she too had doubts about the July 25 phone call, which they both monitored from a separate room. "Do not worry, I will be fine for telling the truth", he said. He added that allegations of corruption involving Biden and his son, who was a director of Ukrainian energy company Burisma, were "not credible".
Col. Alexander Vindman arrives to testify before the House Intelligence Committee.
"I think the Ukrainians gradually came to understand that they were being asked to do something in exchange for the meeting and the security assistance hold being lifted", David Holmes said during the deposition.
Democratic Representative Jim Himes asked Vindman: "Lieutenant Colonel, would you call yourself a Never Trumper?" He told lawmakers that "character attacks" against public servants testifying in the impeachment inquiry were "reprehensible".
And the Republican counsel appeared to question his loyalty by repeatedly asking how he was invited three times to become Ukraine's minister of defense - offers Vindman called "comical" and said he "immediately dismissed".
Republican U.S. Representative Francis Rooney told reporters: I don't think there's been a crime proven yet.
They are the first two witnesses sought by minority Republicans to testify in the televised public hearings.