What I meant was whatever the correct gasket is, how often should they be replaced. Thanks.
The gaskets have an tendency to deform after being torqued down....But sometimes you can reuse them one more time if they seem to be in good condition still. If you have to scrape it out of the hole or pry it off the drain screw.. I would definitely not reuse it. < Also it helps if you're the able to keep an eye on your drive at home for oil leaks but if the boat sit's in the water or somewhere else where you cannot monitor it, it's best to use new every time/year.

O-rings aren't the best choice but are an option from Merc.
Either you can use the blue gasket part number 12-191833 or the new better yellow gasket 26-830749.