I have a 1994 Alumacraft 17' boat. It has a 5/16" x 4'6" seam opening at the front of the boat. I have removed the old seam filler/sealer because it is leaking. Any suggestions as to what kind of filler/sealer I can use to fix the crack/leak?
Hit it with a green scotch-brite pad to clean and scuff the surfaces, give it a wipe down with alcohol or something similar, and use the 3M 5200 suggested above.
I have never seen a 5/16 gap in a hull seam without having some sort of damage or many missing rivets.
Certainly not on an Alumacraft, and we've had several over 45 years old.
It sounds to me that you must be missing the connecting component, sometimes called a chine, sometimes called an "angle" but there must have been something there before.