The surface upon which that clamp arrangement sits is called a rudder table hereabouts. The rudder shaft is held in place vertically (so it doesn't fall out the bottom of the boat) by something clamped or bolted to the shaft which in turn rides on the table or a bearing on the table. That bronze collar with the hex nut appears to the that securing device. Maybe that plastic is a slippery bearing surface. Anyway, you get the idea that you may not want to loosen that bronze bolt. The plastic disk and clamp affair, it is new to me but as suggested by Dave above, may also be a part of a shaft sealing arrangement. We clearly have not yet heard from the expert on your boat yet. It makes a lot of sense to me to have the tube encasing the saft got all the way to the hull and have the packing up higher and under a lot less pressure. I think you will find that to be the case when you look under that flooring. It would be interesting to check to see if your rudder table is higher than your waterline (I bet it is) so that only under certain circumstances you get water through there. I recently got to thinking about my own situation with this small 30-footer. So I opened up the conventional packing seal (above the waterline) to find that there was no, and likely never had been, ANY packing in there. I cleaned up the bit of corrosion in the area from earlier leaks over the years, applied silicone grease to the now shiny metal, and called it good. I am sure this seal occasionally leaked like when I come down off plane and a mountain of water attacks the transom contributing to unwanted dampness in the laz. Maybe you have that situation.
