How much is it leaking. It should drip slightly when the shaft is running but not when stopped, that a good way of telling it is correctly tightened. If you overtighten it till there no drips you may grove the shaft and overheat the bearing. (this assumes it is a traditional bronze type stuffing box. If the drip is a lot more than about 1 per second look to greasing, do you have a greaser fitted and give it a dolop aboout every 6 hours? Failing this how old is the stuffing. It goes hard and needs changing periodically like whenever you do a winter haul out athough some seam to happily last years? If this doen't helt put up more details of your setup and problem.
If it is a long keeler then the tube should be screwed into fittings at each end which are then bedded in sealant if this is the case it is a matter of unscrewing the outboard end and rebedding not forgetting to seal the threads of the tube.
Best of luck