It is very simple and should be done at least annually. There is a cover plate that is commonly held in place with two screws. The plate fits around the chainplates on the deck. Remove the screws and tie/tape the plate up and out of the way (no need to remove the shrouds). Scrape away all of the old sealant and clean the area thoroughly. Lay down plenty of new sealant (DO NOT use 4200 or 5200) and rebed the cover plate.
As far as sealant goes, I recommend 3M 101.
Also, if you have a leak, it may have compromised the core. If that's the case the job will be more difficult. While the cover plate is off, see if the core is wet or rotten. If it is, you'll need to excavate the core around the chainplate, fill with epoxy thickened with colloidal silica, and rebed as described above.
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