There are numerous underwater seam compounds that do not use lead. Much
more important than having lead in the putty is that the seam be properly
prepared and caulked.
Why play with lead when the new products work well? The super modern poly
caulks are usually not necessary but if the seam is dangerously big the
additional adhesion can help. Best is to stick with canned underwater seam
compounds.

I have done a bit but am not a caulker. (find one and ask or read a
book!)
(a) get the seam clean and dry.
(b) prime the seam with paint (most any oil paint)
(c) caulk seam (cotton if small/ oakum if large)
(d) paint over the caulking
(e) putty over the caulking to less than flush (allow swelling)
(f) paint over putty

best
George