in the automotive world, that is a fatal flaw. Automotive V-8's also run a pressurized cooling system much hotter than the marine versions run. The marine engine runs cooler, and also has an open cooling system, so your leak is just a leak. What you have there is freeze damage, and is going to attack the weakest place in the block, be it on the outside of the cooling jackets, like yours currently is, or the cylinder walls which will leak to the inside of the block, which you do not describe (clean oil). You may or may not have smaller cracks in the cylinder walls that have not manifested themselves by allowing water into the insides of the engine (where it does not belong, and will cause more damage). If this were my boat, I would probably epoxy up the cracks, its just an external leak at the moment, and start looking real hard for at least a comparable short block to replace it with. If in the interim there are any signs of water inside the engine, boat gets parked. Cracks get worse, boat gets parked..... you see where I am going with this????
