If the seam is narrow, regular west systems will work to fill the crack. Might run a knife in the crack to get out any gunk that will keep the epoxy from sticking. Flood the crack with acetone to remove as much oil from the teak as you can. May take a couple of tries to fill the crack completely as full strength epoxy will try and run out of the crack if it can. Use a syringe to squirt the epoxy into the crack.

If there is any flexing in the platform, might try there new elastic epoxy. Haven't used it but supposedly works great on problem areas that would benefit from a flexible filler.