Ah, thats where you went wrong then.
The connection is at the very bottom of the incline, sharp left into the neck deep water.
Once you're out of the water you just keep going straight along the Black Cross-cut until the only option is a hole in the floor on the left.
Down that, then under a not-so-obvious ledge on the right.
After that you're back into decent passages and can more or less just follow the flow of water out.