There are good owners and parts manuals for boats for about the last 20 years at the bottom of this page https://www.searay.com/us/en/owners.html
Looking there it doesnt seem to be one for the 2012 240 SD. The 2011 parts manual is here 2011 Sundeck 240 Parts Manual They dont change much year to year, so this might be good for your boat. Looking at it for the gasket, page 33, item 9 lists the nav lights and replacement bulb, but does not list a gasket 😞
