Hmmm, that seems odd. Our hardware stores here have a wide range of plastic and rubber hose sizes. They are all in inches, but usually there are sizes in inches that are close to any metric size. At least you should be able to get close enough that a hose clamp could make up for slight differences.
In a lot of modifications I've had to do, I've sometimes tapered the inside of plastic hoses to taper the inside leading edge, to get the hose on a nipple that was a little too large. I've often been comforted when I've had to taper the inside of a hose to get it on, because you know, that if it is that hard to get on, it isn't likely to leak.
If you are in the USA, contact Ward Richardson or Todd in the Parts Department at BeneteauUSA and tell them what you need. Alternately, contact Raritan in Ft. Lauderdale and tell them the size "SaniFlex" hose you need
Trident marine makes some sanitation hose in some in between sizes, series 148 Most of the heads use the 1.5"/38mm size the water supply does not need to be oder barrier sanitation hose. the PVC sanitation hose can be heated on the end to make it easier to get on the fitting. What size are you looking for?