Answer: caulking

AnswerCrossword Clue
CAULKINGWatertight material
caulkingStop up (the seams of a boat) with oakum and waterproofing material, or by driving plate-junctions together; make (a boat) watertight by this method
caulkingthe material used to caulk
caulkingSeal (a gap or seam) with such a substance