The fix might be as simple as tightening the screws a bit. The washer / cap is the seal. I have had several of the nuts vibrate completely off after a year of hard use, and found many more that were loose. I removed them all one by one and put a drop of blue locktite on each. No problems now.
As far as sealing small annoying leaks, try Captain
Tully's Creeping Crack Sealer. It wicks into small cracks and seals them permanently. You can order it from marine supply houses.