This must be a dealer day. First the cost to service an outboard, and now the cost to fix a coolant leak.
I'm no expert on what will ruin an engine, still experimenting, up to about engine 10. For the >$3000 price of a new engine, I'd have 2 suggestions.
1. Go to your trustworthy independent garage, tell them the problem and ask how to fix the problem. I've spent 7 years in Escondido finding a good garage, and I've finally scored. They're hard to find, great to use.
2. Get the problem fixed. The 350, the 305 and the 283, etc. are basically the same engine. And yes there's a couple of coolant holes in the block/intake manifold. So, fix it and see what happens. What do you have to loose? The bearings are probably OK, since water and ethylene glycol will lubricate a bearing, especially when mixed with oil. Remember the oil salesman with the "special oil" that he'd mix with water, run engine and tell you how good the stuff was? As long as he changed the oil after he was out of sight, the mixture would lubricate for a short period.
In defense of the dealer, he had no idea of how long the coolant was in the engine, etc, etc. With your money, the safest recommendation was a new engine.
Boris