GS- looping the reins is how I was *taught* to ride out of the water and has helped me with a couple of other horses. But, I'm gun shy now since that's how I rode that rolltop in the water last year...boldly forward with a loop. It may just be that Toby needs more education and time.
That might be your problem. I was taught to never loop reins in the water, to always keep a feel of the horses mouth. They need that extra support of the reins to help them keep their balance with the drag on the water. Think of it this way. If you had a drop, 5-strides to roll top without water would you look the reins? Most likely not, so why would you do it in the water? I can completely see how a gung-ho little horse would misjudge the roll top out if he had a loopy rein in the water. Keep a feel of his mouth so he can remain adjustable through the water. Any time you ride on a loopy rein you loose your connection and adjust-ability. And the water jump is the last place you want to loose that.
Never attribute to malice that which is adequately explained by stupidity.
That might be your problem. I was taught to never loop reins in the water, to always keep a feel of the horses mouth. They need that extra support of the reins to help them keep their balance with the drag on the water. Think of it this way. If you had a drop, 5-strides to roll top without water would you look the reins? Most likely not, so why would you do it in the water?.
This is a valid point - generally - but two caveats which makes it inapt here. First, while I wouldn't loop the reins in your drop situation, I would have quite a soft rein that encourages the horse to pour itself over the drop, rather than launch. Looping is an exaggeration, meant to try to identify the issue here. Knowing how many horses I've had (particularly OTTBs), who will go at the pressure of a tight hand, I am guessing that this particular horse is doing some of the same thing, perhaps in reaction or anticipation of getting constricted or caught in the mouth. Thus, if I was OP, I'd try seeing what happened if I was very careful to have a soft soft soft hand, maybe even a loop, and I'd be riding with a neck strap to ensure that I never ever caught him in the mouth over a fence or a bank.
FWIW, the "loop the reins out of water" thing I learned from Lucinda Green ages and ages ago. And it has helped a lot with Ralph and Vernon (both tended to stand off a bit at banks out of water if held), and couple of other horses over the years. I don't know how well it works on Toby....but, then again, the one time I very consciously made that decision, I got very wet. I will admit, wholeheartedly, that I am more than a little gun shy!
So, we had a big water school today and I learned some interesting things about myself (and also nailed home why I HATE schooling xc on a good, going horse....having a little show day adrenalin makes a WORLD of difference for some questions).
The biggest thing I learned, which is true in a lot of my riding but makes a HUGE difference in the water, is that I hesitate and wait to see what's going to happen IN the water. Every time I did that, we tripped up the bank or jumped out awkwardly or otherwise mucked it up.
I do have to take the time to be sure he is soft in the bridle BEFORE the in to the the water (this is the story of our life, but makes a big difference in the the water). While we nailed every single one of our entrances (from galloping straight in to a big jump/drop in), if I didn't have him soft in the bridle BEFORE we got in the water, it was almost a sure bet we would jump awkwardly out.
I need to hold on but kick. If I chucked the reins (or dropped them), we would jump out awkwardly. If I pulled and didn't kick, I'd encourage him to launch. If I held (while trying to keep him soft), but kicked like a mad woman, he'd add up and jump out nicely. I also had to make a point to say WHOA on the first stride in (easier said than done in our water, which is only 2 or 3 stride across in most of the combos).
It was a very educational school, even if my heart was in my throat through some of the combos (MUCH rather jump a few of those questions in competition rather than schooling!). The best part was feeling him "get it" at the end. Our last time through I could feel him think about leaving a stride out as we left the water, but he waited and shortened and jumped out at the base fantastically!
Now to see if I can remember how to do that this weekend!!!