Plastic and rubber may not melt anywhere close to 110, but sustained heat can shorten the lifespan of any material except maybe metal. Remember too that cars don't have bilges... if/when the coolant or washer fluid reservoir fails, it's only a puddle in your driveway....in your boat it's a mess in your bilge.

So a limit of 110 may be low to keep it on the safe side, but high enough to keep tanks and plumbing out of most engine rooms, where temps can be much higher and the only place on a boat where temps are likely to be even as high as 110.

However, it's prob'ly not a problem on most sailboats, 'cuz sailboat engines typically only run for brief periods (at least that used to be true anyway)...unlike trawlers and other powerboats whose engines and gensets can be running all day. That's a LOT of sustained heat!