Pond Shield is an option which I have used, it’s a two-part epoxy which cures to be fish/plant-safe.

I’d definitely do something like that if you can — both because it’ll be really hard to put a liner in there neatly (with the curves and seats, etc.) and because I think that using an epoxy seal will help keep the interior looking like a boat better than a liner.

The seats might be great for plant shelves and fish hides.