I think you nailed the "watery" soundscape that you were going for in this soundtrack overall.
I wasn't the biggest fan of the addition of the drums in "Waterways". It's such a sick ambient piece, and the drums felt a bit out of place and unnecessary.
"Swimming Upstream was a very interesting track. It branches off from the overall tone of the soundtrack a bit more drastically than any of the others do, but still remains cohesive. I do wish the melodic elements were made more of a focus than the low panning noise layer - but I can see what you were going for.
"Sunken Castle" is by far the best track of this entry. Every element fits, the transitions are smooth (when the drums cut and the bass comes in, and then the drums come back - DAMN). Sound choice in this track was incredible as well.
"Plummet" is 50/50 for me. I enjoy the sound choices in this track. The addition of some more distorted sounds was great, and added a unique character to this track. The drums in this one I feel could use some work. The level of them works well in the chorus, but in the intro the are quite loud and don't feel placed in the mix well. Adding some little fills to the drum loop in the chorus would have been great too, as they get a bit stale.
Overall this entry is great. I enjoyed the cohesion of the "watery" soundscape throughout the whole thing and the sound choices were wonderful.
Keep up the great work!