We know Stannis had a semi-large fleet when he traveled to the North and went past the wall to defeat the Wildling army.
After that, Jon Snow used the fleet to save the Wildlings at Hardhome. Stannis did say he expects his ships back because he will need them to conquer the rest of Westeros.
We all know how that turned out...
Stannis' death is pretty much confirmed in season 6 when Brienne of Tarth tells Melisandre and Ser Davos that she executed Stannis personally.
Does the TV series hint in any way what happened to the fleet and its crew?
The ships can’t be used to take back Winterfell so it’s normal that they would not be discussed during this period.
Still, such a fleet could be a powerful tool in the right hands.