Interior liner ??
(0) I assume an interior liner.
(1) Think like a drop of water. It is very possible that the actual leak can be a long way away. If necessary as a last resort, drill very very small holes in the interior liner where water could be puddling in the hidden space above on the way to your 'trouble spot", in order to determine the drops path.
NEVER try to seal leaks on the inside/bottom. All you do is cause the water to back up/puddle and evenually leak elsewhere and meanwhile the moisture causes corrosian and decay.
ALWAYS seal leaks on the outside where the drop enters the hull/deck.
NO job is too hard for the person that does not have to do it!