Check to see if there's a grease nipple or Stauffel cup, if there is, use grease gun for 5/6 strokes, half turn the shaft and 5/6 strokes. If a Stauffel cup unscrew it, fill with grease and tighten. Run the boat in gear
for approx. 5 mins, check for leaks
, if still leaking tighten each of the 2 nuts on the stern gland half a turn alternately until the leak is down to once every 20 seconds or so.
I would suggest you check it on a regular basis in future.
An alternative replacement to the seal you show is a PSS seal, I can highly recommend them from practical use.