I have not seen your paddle style, but on my canoe paddles, I have a plastic blade with an aluminium shaft going in it. Once the shaft is inserted in the blade, it's riveted there.
On mine, I drilled out the rivet and removed the blade to drain it. Once drained, I applied a little bit of silicone on the shaft before inserting it into the blade. Put a new rivet or SS sheet metal screw.
For blades that have never leaked, a small bead of silicone where the shaft enters the blade and over the rivet ought to prevent a leak.
1978 Hobie 16 Keoke, sail# 36 84