Since the fiberglass / crack moves when you press on it, it's a pretty good indication of something that, at least, will become a major issue at some point. Depending on what structure is behind that area (I can't imagine there is anything at that exact point), you can probably just do what I did with my cracked seam...grind away the glass until the fiberglass is paper thin at the crack and then taper it outward. Then fiberglass in that starting with thinner strips and working out to the widest strip that covers the entire area (and then a little more). Sand / fair it, paint it gelcoat it, and you are done. If there is a bulkhead behind that area, repair will be more complicated....all this really depends on what caused the crack in the first place - if it's a construction or design problem, you need to do more than simply fix the crack.

That said - is the core of that boat balsa? If it is - you REALLY need to at lesat seal up that crack before (more) water penetrates and destroys a large section of the core. Since you can move it I would be concerned about water penetration.

Sorry to sound so gloom - we have to kind of assume worst case scenario since we can't see it.