Just testing the mod for the first time, which I love the concept BTW. The furnace and chest boats disappear from view when more than about 16 blocks away. Can replicate this on both water and land. And I turned on chunk borders to confirm it's not specific to being within the chunk, but you can walk away and it just disappears. Then when you return within a 16-block radius the boat(s) re-appear.

This is on boat container version 1.1.0 on 1.18.2.

Tested on the latest Boat Container Version (v. 1.2.4) on 1.18.2 and this bug is still present.
I place on line every single boats and on 16 blocks away they start disappearing.
I don't know a lot of modding but my suggestion is to set an option on the config file where you can set the distance of the spawn radius of the boats or something like that (like boat-render-distance-view: 4 chunks).
I think It will be good for both clients and servers.