All marine blocks using raw water cooling have rust in them
However I'd you want to flush it. Mix up some oxalic acid and fill the water jacket up.
Let sit for 3 days.
Flush with water
Repeat
Are you sure about the part number?
Per the parts breakdown at https://www.volvopentastore.com the block drain is 3854494, https://www.volvopentastore.com/Cylinder-Block-And-Flywheel-Housing/dm/store_id.366--view_id.790335...
Get a tow to the dock
Find a new mechanic
Pressure test your engine cooling system
If you are lucky and it's just the manifolds, buy a lottery ticket
Good luck on finding manifolds
My first thought is the engine is running hot. With your new to you boat. Make sure you do all the maintenance the prior owner didn't do
Like the impeller service, annual drive service, u-joint inspection, alignment check, etc.