I think that you will find that you are being unduly pessimistic regarding the use of uPVC pressure pipe given that it has a service
life of at least 50 years (typically 100 years is quoted for it before rehabilitation in water and waste pressure pipework). On land PVC is used in much more onerous services such as for gas pipework. You mention it as being a cheap
plastic - in my experience uPVC pressure pipe and its fittings are not cheap. Perhaps you are thinking of the like of uPVC domestic waste pipe and fittings which are cheap?
In the case of a yacht main engine or generator
below the waterline as long as it is protected on the raw water
suction side by an anti siphon loop and the exhaust discharge is overboard
then the only possibility of flooding is from the engines own raw water
pumping, not from back flooding from the sea.
I would suggest that the likely risks of that are higher from failure of pipework, joints, seals
on or about the engine, including the water injection elbow
and riser to the water lift muffler, or from failure of exhaust rubber hose (whether used for joints or runs) or water lift mufflers such as or similar to the Vetus ones commonly fitted which have a reputation for failure (the likes of Centek excepted) than it is from a properly installed uPVC pressure pipe wet exhaust.
Also, the materials commonly used in exhaust systems all have their problems including the hose that you recommend. As an example, I recently project
managed the build of two big power boats for a client and which were designed and built to Lloyds rules (SSC). The main engine risers and exhausts (exhausts approx 8 inch) were 904L ss pipe to aluminium overboard
penetrations - I can guarantee the life of that high quality solution will be much less due to corrosion
and fatigue than that of smaller diameter uPVC pressure pipe properly installed in a small pleasure vessel.
In the case of the generator
exhaust in the original posters question, if it were my problem I would take the exhaust from the through hull
(which is only 3 inches above the water line and is also subject to flooding during heeling so cannot be classed as an overboard discharge on either count) from a valve at the penetration, which it not being able to be classed as an overboard discharge it should be fitted with, to 300mm above the maximum heeled waterline in ss or grp pipe, in reality probably to the top of the anti siphon loop mentioned. I would not use a lesser material such as rubber hose (nor uPVC) for this section as back flooding is possible if it fails even though I would expect the likelihood of failure here due to raw water cooling
loss would be low because of the long run from the muffler.
Personally, and only personally given my experience with it, for the rest I would use uPVC pressure pipe but grp would be preferable - however, given the long length I would not like to be the one paying for running it all in grp. I would rate both those, properly installed, as being significantly more reliable than approved rubber hose which I would personally never use in an exhaust for anything other than flexible joints.