This has probably already been mentioned before, but I'm rolling the idea around of developing a 2d cutaway ship building/management game. Being able to build and install different ship systems with the ability to upgrade systems as you progressed. Sort of like FTL mixed with Starbound, but more focused on the ship management and being able to build rooms and placing objects however you want. The shipbuilding part isn't the hard part for me, its more of how to make a game like this fun and with purpose.
Sounds like you want something that works a bit like
SimTower.
I could imagine that you'd start with a "shell" hull of a chosen size and then place (with various restrictions, based upon locations on location) engine-rooms of various capabilities, cabins (port-holed/windowed ones only certain distances above the Plimsoll Line), access point (again, above water-level), perhaps a "glass bottom" room or a sub-dock (below, obviously), and funnels and bridges and the like at the top (the funnel(s) also needing connections to the engine-room(s)). Crew and (where required) Passenger lifts/stairwells need placing. Closeness of cabins to the engine-room creates bad feeling amongst those so assigned (annoying for crew, perhaps less so for the engineers than the top-brass, but enjoyment-destroying for paying passengers if you're making a pleasure-liner). Leisure facilities will range from simple crew-rooms to deck-top artificial surfing pools for fee-paying passengers. Simple galleys would be required, and perhaps far posher kitchens adajcent to the luxury restaurant room.
Of course if you're designing for cargo then it'll be crew-only (perhaps with some 'hitcher' room) for most of the facilities and a lot of the shell (and shell-top) space will be specifically cargo-holding space. Your luxury cruise-liner will balance in the other direction (storage for stores, rather than for in-transit, but room for bulk passenger).
Perhaps you just want to design for a simple purpose. Or perhaps the game allows you to choose your ship's destination and destiny. Going on a route past Mozambique (or some fictional shoreline that is equivalent), and you need to add anti-boarding measures such as razor-wire along the walkways and patrol-posts for armed crewmen to spend time in on vigilant watch. Spend money on armed guards, and/or on training your existing crew in defensive practices. Your luxury liner could earn bigger bucks when the government enters a war and is willing to pay a premium for a troop-transport (some refitting necessary to make luxury cabins into increased multi-occupancy for units of squaddies, maybe repainting the sleek-white lines to battleship grey), and again features for ship-protection (ASDIC? Anti-missile automated Gatling guns?)... And once you return to civilian use (presumably having survived the tour of duty, but maybe with damage needing dealing with) you need to "posh it up again".
Periodically better engines/drive-systems/kitchens/entertainments can be purchased and fit into the hull (replacing the original equipment, or adding to it to make the even
more luxurious dining experience with the stage show feature... needing the hiring of appropriate entertainers along with the usual hiring-and-firing process for the below-decks or above-decks crew), giving new values for speed, fuel-efficiency (don't forget the fuel-tanks), self-sustenance (water recycling/desalination, later maybe hydroponics for fresh vegetables even on the longest cruises/missions), self-defence, storm and weather resilience, etc, etc. Some of the additions can be done by merely refitting (freshening up cabins), others would require being taken off-duty (engine replacement) and perhaps even dry-docked.
For a
really big refit (assuming you can't sell the ship, either as a gutted hulk or a going concern, then buy commission yourself a new empty/partly shell) perhaps you can pay heavy wonga for dry-docking your vessel for a longer term and extending the shell in various allowable ways (lengthening sounds easy enough, raising the superstructure height sounds like you might need to simultaneously deepen the keel, or at least suffer an upwardly-moving waterline and thus converting the lower port-holed cabins into being windowless ones... and either way this might have impact on which ports you can easily service, or at least whether tides cause restrictions on arrival/departure), thus giving you the room for more cabins, storage or other features.
Apart from the piracy/military dangers (if you want to implement those) I imagine you could also have to deal with heavy seas, icebergs, other natural dangers. Also the wearing down or breaking down or destructive damage (if not maintained well enough, both in-use by the resident engineers and at scheduled dry-dock stop-overs) of ship features. You could keep your costs down by running your rusting hulk of a cargo ship (at least you don't get any passengers complaining), but you'll probably lose out if cargo gets knocked overboard in a swell and of course if the ship is sunk then if it's not automatically end-game then you're in a "salvage mode" situation at best. Did you get enough insurance at Lloyds Of London to cover that process?
Oh, I can see this game being really rather simple (just cater for general mission demands, much as with SimTower) to being a quite complicated multi-plottable management game with changing external influences (mostly according to what routes you're servicing, and across what expanses... need not be "real world", so long as you get "expected to pass within 50 nautical miles of a Pirate Paradise" or "this route needs you to navigate a canal between oceans" or "the destination port has a low-tide channel depth of only <so many> metres", and of course the distance required and speed you can travel so you can ensure you have enough supplies).
Perhaps have the opportunity to maintain a
fleet of boats (once your original barge earns you enough money to create a ferry, or a Mississippi gambling riverboat or a floating crane platform (earn money for supplying salvaging capabilities to others, or lets you rescue your own craft when things go wrong!) or an oil-tanker... Or even oil-
platforms? And if you get a selection of hulls to choose from I'd quite like to see (at a premium) a
submarine shell that you can equip. Perhaps an advanced challenge is to make it a
cruise-submarine... Paying guests. Pressure-capable viewports, diving pools for Scuba (or Alvin?) expeditions, deck-features that can survive submersion but can still be entertaining when you surface according to schedule (weather conditions allowing). Extensive 'indoor' entertainments for the rest of the time.
There, a lot of scope. And because of that I imagine someone has already done something within the phase-space of this suggestion, but (despite being a management-game fan) the closest I actually know of is the entirely ship-less and quite lacking in water thing that is SimTower, still.