Yes, boats of a certain size (10m minimum, if I remember correctly) are considered “well-mobilized registered goods,” so the ownership transfer must be done in front of a notary.
I don’t have the exact amounts, but I can find them if you want.
If you plan not to keep the boat under the Italian flag (which would be foolish, in my opinion), the best thing to do is to negotiate with the seller to cancel the Italian flag and register it under your preferred flag.
For example, if you want to register it in France, the seller will request the cancellation of the Italian flag, and once obtained, they will ask for registration in France (free of charge, and entirely possible for an Italian resident—at most, you could lend them your address for the registration), then a sales contract according to French law. These fees are comparable to the cost of a postage stamp.
Now that I think about it, you’ll need to check for the DAN (tax), because if they register it in France, even if it’s just for a brief moment, they’ll owe the tax. But you can work it out with the seller—it’s an annual tax, so if they pay it, it’s not your responsibility, and vice versa.