Kept waiting for that reveal myself. Even re-watched the last episode as when I read Jason & Bryn were going to put their secret to bed they meant a complete reveal. No luck as I saw. They referenced it so often, you'd think they'd have slipped us something more definitive.
It appears we are meant to speculate. With Bryn's naivete, it could be something that most of us consider pretty innocent. But it does seem to be implied that something sexual happened between Jason and Bryn, or if not sexual, then homoerotic. There are several illusions throughout the series that Bryn is gay and either isn't aware of it consciously or is in the closet. And we know that Jason is gay. So a reasonable inference one could draw from the clues that have been given is that there was some type of contact between Bryn and Jason that crossed the line between uncle and nephew.
I'm only at the middle of series 2 so I came to this board to see what happened in the end or to read what people thought had happened with Jason & Bryn. I posted pretty much the same speculation as MikeGrossman and others in a similar thread earlier today.
I especially agree that it's likely the sort of thing most people would have just laughed off but, Bryn, being so odd and naive, made it into a dark secret.
The writers themselves don't even have a conclusion for it. If they were to make another episode and no doubt 'reveal' what happened on the fishing trip, I get the feeling it would be a huge anti-climax and something so stupid it wasn't even worth knowing. Some things are better left unsaid.