Maybe this is a personal bias, but I find changing the fidelity / realism of the instruments a little jarring for descending the depths of the ocean. Which is strange when I think about it, because the bottom of the ocean could resemble a different planet or dimension with the lack of sunlight, strange lifeforms and ridiculous amounts of pressure. I don't find a change in time signature, key signature or instrumentation that weird for some reason. I'll need to think about this a little more to figure out why I feel this way.
Good solution anyway, using a track to transition between the two sections would make it feel more natural.