Is it reasonable to use silicone hoses tested at 7 bar for the cooling water pipeline between the stern, the impeller pump, the heat exchangers and up to the risers?

What brand and model of hose are you suggesting?
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The Wonder-Flex style marine hose is hard to beat. Smooth wall inside, and capable of reasonable corners without collapse. Most sizes good to 200psi.

https://races-shop.com/105-silicone-hoses-and-clips
They are tested with 100 psi and they have 4 layers of poliester fabric.
https://turboworks.pl/product-eng-16...35st-28mm.html

i only use wire reinforced marine water inlet hose with the coast guard stripe and date on it for below the water line thru hull fittings its required

The impeller pump inlet hose can get a significant suction within. You want something that isn't going to collapse.The orig Merc hose was a very stiff hose with wire wrap built in. A real PITA to work with, but needed. For this hose in particular, I would stay with the orig Merc hose or something similar.

That`s not a good idea. Ive used those ...They have no reinforcement , kink easy and might slip off easier.

https://turboworks.pl/product-eng-16...35st-28mm.html But there it is 4 layers. Factory claim that they are tested for 7 bars and I am put 8 bars of air in one elbow. It was hard like stone, but it is survive.

After this short discusion I am convinced, thanks for help!
For raw water I keep classic marine materials, but for close cooling system will use this silicone hoses. (I have marinised LS376/480 and it is wery easy to made nice coolant piping with this fittings.)

Ive used them in a few places

Dan how has the hydro dipping held up on the manifolds?

I only had it on there for a couple years , It held up pretty well except for the bottom of the flange where it`s the hottest but that can`t be seen.