I suspect the advertising here is supplied by a service that the forum owers have contracted with. As such, the selection of ads is determined by the service, not the forum owners.
Usually in these cases the ads put on forums, websites, etc. are tailored to the demographic the forum, website, etc. most appeals to. So boat owners have been deemed to be in the demographic which would be interested in Jeep vechicles. Hence the ads.
Given the average age of cruising boat owners I wouldn't be surprised to see AARP ads appearing here soon (if they haven't already--- I totally ignore the ads to have no real idea what's been running here).