Look into “bear poop.” It was used on the west coast for years to do commercial boats, its a mixture of portland cement and latex rubber and every wooden tug on the coast used it. Call one of the local commercial yards here and ask where to get it.
Here is some “local knowledge.”
“...On others, a long spline cut to a wedge shape has worked in repair to glued-edge boats like Knarrs and IODs where a seam had opened, to restore the glued-edge structure.
An eastern Canadian method is cotton-caulked seams with routed-in battens over instead of putty.
In heavy planking, such as tugs, the old common method is no putty in the topsides, just pounded full of oakum, and portland cement as putty in the bottom seams over the oakum.
West Coast caulking style for over 1.25" plank is a choker of cotton, caulking the job complete, then a line of good soft long fiber oakum over that leaving a seam about as wide as it is deep, then red lead paint, then topside seam putty or underwater seam putty as the location requires.
"Bear S**t" is a NW USA seam putty mixed from powdered Portland cement and black roofing patch tar, applied below the water over the caulking.”
“Start with the second seam down in the topsides. If you over-caulk your 'learning' seam, you'll push the sheer strake up and off the boat.
The proper irons are crucial and you only need two for this job, a very thin OO size that you loop the cotton in with in 'tucks' and then caulk it in the seam with, and one about 1/8" for 'making' the cotton or tamping it down evenly.
The process starts with the butts first, leaving a 1" tail hanging out anywhere you stop, then the seams, crossing the butt tails and trapping them in. All the seams run out at the stem, again with the tails, last seam is the 'devil' or stem rabbet/garboard which traps all the plank tails.
After these steps you're there.
The proper caulking mallet (eBay, searches "caulking or caulker or caulk") helps, but for just one boat, a square wooden carpenter's mallet will serve. Not a round one, you can't control the blows right.
Here's a couple places to look.