there are three screenshots here. The first two are made with F3+A clamped, the last one without. The first one with the mod disabled, and the second one with the enabled one. As you can see the boats are always rendered

I think I put boats and minecarts on an exclude(maybe under certain conditions). This shouldn't be an issue.

on servers, boats are actively used for movement in nether. I think the choice of behavior can at least be added to the mod config

This was done to prevent bugs, so no config value.

Looking at it again, boats are part of the config for tick culling. For rendering they don't have anything special(anymore?).