Google my business has separate categories for "Primary Business" for each site:
Each site gets the advantage of having its own unique Google knowledge graph that area that shows up at the right of a search for a company. And Google my business, listing which feeds Google map and the knowledge graph.
https://support.google.com/business/answer/7249669?hl=en each of which has benefits for businesses in that category. IE:
A hotel has free online booking an gets listed in the travel pages, https://support.google.com/hotelprices/answer/10684696?hl=en
Restaurants get online menus, https://support.google.com/business/answer/9455840?hl=en
There is a category for wedding venue.
https://daltonluka.com/blog/google-my-business-categories and Boat dealer but I don't know if Google has any special knowledge graph information related to those. Although, if Google knows the person is interested in boats, and wants to give that person the information they want, (which they do), having the site with boat dealer as the primary business for Marina boat sales should help.
To be a site with a Primary Business.
One could use a subdomain for each primary business ... EI marinaboats.example.com weddings.example.com and Restaurant.example.com ... But there is no SEO advantage to doing so, and what do you put on example.com. Sure that makes some sense if example.com has investor relations etc.
The cost difference for having separate sites is just the cost of the DNS service $10 a year, less than $1 a month; And, having the URL match the business's name is helpful for SEO. They can all be hosted on the same host regardless. Also, registering a domain with the name of the business stops others from doing so.
Google keeps adding services to compare
The services Google keeps adding to compare businesses are rich spaces for getting traffic from Google. If a site's primary business is boat sales they don't have menus in the Google my business section or if they are a wedding venue no 30-day guarantee on parts and labor ... and no data means no comparisons in these extra google services.