for things like that i use a broken sawblade, the ones for metal work well, file the non toothed side at an angle so it looks a lot like a small and slim paring knife, and sharpen it, once you put an handle on that the tool is ready.
Its used pushing the tool into the wood, the fact thats like a stiletto lets you push it quite far if not all the way through, everytime you pull the saw side makes the cut larger letting you push further in, once you are through you just use it like a saw, cut along the joint line, glue and re-clamp in place (now that i come to think about it its nothing else than a sort of really small and sturdy drywall saw). Thats the same tool i use to start the cuts for the cockpit and hatches.
In the pic you show more than the gap is the fact that the butt joint is misaligned, and in an accent strip it can get sort of noticeable, next time try to join the strip first and put it on the boat later, its easier that way. For any other gap a wood shaving pushed in the gap would be enough.
Its a small thing and wont take long to fix it and for sure you will be happy you did now that you can.