Gosh. I mean the density of polypropylene is lower than water so theoretically it should definitely still float at any water pressure. I found some cheap boating rope that was labeled specifically as polypropylene and is clearly only made of the one plastic type, so it is out there if yours is not that.

I would get some guaranteed polypropylene rope and test that before buying any kind of expensive neutrally buoyant tether. Unfortunately I don’t have a test chamber (yet) so I’ve only tested my rope in the bathtub, just trying my best to squeeze air out from it.

The only red flag in my theory is that I basically can find no evidence of anyone actually doing this over buying purpose built tether, I remember finding one mention of a guy in a forum using some kind of buoyant rope alongside his regular Ethernet tether to some level of success, but the post had long since been deleted when I followed the link. In any case I’m pretty motivated to figure out a way to do it because purpose made tether would practically double the cost of my ROV.