A word on the processing side, since you found the plugin makes things worse when pushed: that is not you using it wrong, that is what it does.
Broadband noise reduction works by identifying the noise and subtracting it, and it cannot subtract only the noise. Push it and it takes the quiet detail of the performance with it, which is the underwater sound you heard. It is genuinely useful at a light setting on a problem you could not prevent. It is a poor substitute for not recording the noise.
What is worth doing at the mix instead:
High-pass everything that does not need low end. Most rumble lives below where a vocal starts, and this is free.
Ride the level or use a gate gently so the noise is not sitting there during pauses. A lot of what people find objectionable is the noise floor appearing in the gaps rather than under the performance, and simply silencing the gaps handles it.
Be careful with compression. You noticed this yourself: compression raises the quiet parts, and the noise is a quiet part. A slightly less compressed vocal often sounds cleaner than a heavily compressed one that has been denoised afterwards.
And the general principle worth keeping: the noise floor is set at the moment you record. Everything after that is damage limitation, so the ten minutes spent moving the microphone is worth more than any plugin.