I think if frontier will want to add underwater tunnel it might will be the best in he walkthrough exhibits, if frontier will add like shark the best solution is to use the walkthrough exhibit because those animals they not like doing so much so if frontier would want to implement underwater tunnels i think really the walkthrough exhibit would be the best for it.
of course it will need different shapes and sizes and stuff like that (what I hope that will added to the regular exhibit!) but that's what I think would be the best if frontier even will want to do something with underwater tunnels.
just look at this pic I don't if I'm the only one but if frontier will added it, I think that's how it gonna look.
of course there are animals like penguins, otters, seals and more... that would be so cool to see them in underwater tunnels, but I feel the walkthrough exhibit It's the best solution for the game, It's the same as free flying for example I think the reasons why frontier implement flying to the exhibits It's because the fact that just think how the game would be laggy after that if you having a big zoo and then frontier wil added a free flying just think how your zoo would be even more laggy, so I think really the walkthrough exhibit It's the best solution for birds and if frontier will want to added animals like sharks, fishes, rays.