The best response to this, in a situation where you have no intention of running "Merchant Ship and no adventuring", is you say "Very well. You don't need me to DM for that. Get back to me when you want me to run you through Caverns of Thracia."
Sidenote: There are systems that revolve not around adventuring. Take Prime Time Adventures for instance; I've seen PTA used to emulate sit-coms like "The Office". Something like this might be more appropriate to "Merchants on a Ship", turning it into a drama about the lives of men at sea on a cargo vessel. A possible better fit than D&D. Sitting down to play D&D implies, well, dragons and dungeons.
Yeah, that's my feeling. Now there are rules for playing a Merchants game in eg the D&D Gazetteers The Minrothad Guilds and Republic of Darokin , but basically I think certain activities don't work that well in a D&D game. The players need to be willing to play adventurers. Maybe adventurers who do other things too, but still adventurers.