Ciao Vaz: floatation seem slightly negative (less liters than weights of board+rider+paddle+wetsuit)
You can have a glimpse of the "ratio" by looking at the recording of the live events.
Mo Freitas was definitely sinking to his knees on Day 4 of the ISA event:
I have SUPed lask week mostly on a slightly negative volume board (105 liters for my 98kg = 107kg total: me+gear), but wide (30"), with wide nose and tails (yes... a Tomo shape :-), and I must say it is not as hard at it seemed, much easier than a 125l x 27" board. The low volume was quite helpful in the powerful days at Hossegor, granted the width gave latency to rail-to-rail action, but the low volume made the board easier to handle in the power. But the low volume noticeably kills your paddling speed.
Here are me and a friend, both ~100k, me on a 105l on the left, and he on a 125l on the right
The Pro boards are insanely hard because they are both narrow and low volume.