The point was to show that utopian solutions might sound good on paper but The Devil Is In The Details. The particular Devil in the case of gun confiscation would be the exponential increase in Boating Accidents (Sorry, officer, I was out on my boat with all my guns and ammo and they all just fell overboard. It must have been a freak wind gust or wave or something).

Elimination of civilian ownership of firearms isn't exactly "utopian" - because, again, that type of policy actually exists in several nations. "Utopia" literally means "no place" - the ironic point that More was trying to make is that certain types of fanciful imagined "perfect worlds" cannot exist in reality. But there are actual, real-world examples of countries that have negligible civilian ownership of firearms. If that were something that the people of the United States actually wanted to implement, it probably could be done.

Note that your "Boating Accidents" don't really pose a problem for this policy. After all, the result of that "Boating Accident" is that the guns and ammo are actually lost - and likely permanently. It doesn't really matter if the guns get destroyed as part of a buyback or because they're tossed overboard - the outcome is the same.