>As a Christian myself, nothing that you describe is inherent to theism

Maybe not to theism but Christianity's eschatology speaks for itself.

>To the point, I don't believe that's an issue of philosophical coherency on theism.

It is in so far regardless of whether someone wants to read meaning or meaningfulness into it or not, generally speaking it is within our nature to desire survival (incarnate). Believing in things which are not evidently true (such as theism) may lead someone to act against this inner desire (suicide cults would be an extreme example). If you care about survival like you claim in your boat metaphor then all this should be coherent enough.

>If life is worth clinging to temporarily, why is life not worth clinging to eternally given this chance we have to search for it?

I could think of many good reasons why humanity would want to call it quits at some point. But then at the same time I also never claimed it wasn't worth clinging to and who knows maybe the survival of our species is worth it. But then we still have some 10 billion years until the Sun dies and perhaps 22 billion years until the universe ends and plenty of time left to think, discover, and act if there's something to act on. And if not, as someone who worships the God of Joy, we may as well enjoy it while it lasts.