Add a new chapter to the Mike Vrabel and Dianna Russini saga, because TMZ Sports has learned they rented a private boat together for a day on the water ... just the 2 of them.

The NFL duo booked the vessel in Putnam County, Tennessee back in June 2021 ... according to a source with direct knowledge, who also says the then-Tennessee Titans head coach and the journalist were the only people onboard for their 2-to-3-hour rental.

The timing of that outing might raise a few eyebrows, because Russini was pregnant at the time with her first child.

Documents, obtained by TMZ Sports, show both signed the company waiver before heading out on a lake ... and our source says there was no visible PDA before they departed.

We're also told Vrabel and Russini appeared cautious about photos being taken, and while she declined to pose at all with the staff ... Vrabel agreed to snap one pic, but asked that no images be publicly posted.

Russini gave birth later that summer to the first of her 2 kids after marrying her husband, Kevin Goldschmidt, in 2020. Vrabel has been married to his wife, Jen, since 1999 and they also share 2 children.

A week after the boat outing, Russini posted an IG photo ... seemingly of Kevin on a New Jersey beach with the caption, "Lucky to have great people to spend this holiday with this July."

As we reported, speculation about their relationship started when they were photographed looking cozy at an Arizona boutique resort ... and since then, other photos have come to light showing Vrabel and Russini doing everything from gambling at a casino in 2024 to kissing at a bar in 2020.

We've reached out to both Vrabel and Russini about the boat trip ... no word back yet.