The only definite things I have heard are that Chas, Debbie, Cameron and Marlon are involved in the filming. If Marlon is involved it is quite possible that there are other Dingles involved but that's the only 4 who have confirmed that they were involved in the filming - which I think only finished last week.
The studio that they were filming at does have some underwater sets so it is possible that the underwater pic was a set up for the stunt - although I dont think it was Debbie and Cameron in the pic that was posted.
I think we will have to wait until at least the beginning of Oct to find out what is going to happen.
In the meantime I would be happy just to find out how Cameron