To close the loop on this thread, I checked my octane settings and I was already in direct lighting mode. I guess for me the "fast" render still felt slow :P
I ended up giving up on octane for this project since it's an animation and waiting for long renders didn't fit into my workflow. I used the built in C4D viewport render, with transparency. I unfortunately had to turn off the shadows since my scene was underwater, and viewport shadows render the transparent water black.
I am considering experimenting with u-render. The realtime workflow sound really cool.