If I'm trying to book an airboat ride and it states one left, does that mean seating for one more person or one more airboat going out at that time?

It means 1 seat left on the boat going out at that time.

It means one more seat for that time slot. I just found that out yesterday. 6 left is an open boat.

Its usually 5 or 6 one a boat. But, it sounds like its meaning only 1 more boat for that time.

There is no assigned seating