As might be expected there are very few SF Submarine models available which are of use to the gamer.
First choice, is to use card templates which are quick, easy, cheap, but unfortunately not very visually effective.
Dragon do a range of 1/700 submarines in their Modern Sea Power Series. Currently these can be quite difficult to get hold of, but offer gamers a lot of model conversion opportunities. Various submarines are available, including the following: the JMSDF Yushio, the Russian Delta III Class,
Monogram, who produce the licensed seaQuest DSV range of models, produce a 1/600 scale seaQuest, which is big, very big.
Tamiya produce some World War Two era submarines in 1/700 scale which are not as useful as some of the previous models mentioned.
Various manufacturers do produce a range of WWII submarines (nearly all U-Boats) in a variety of scales around 1/96 to 1/150.
With some extra components, from a bits box, or cut from plasticard, the models can be given a SF feel. Though of course, there is nothing to stop you using the models straight from the box, as due to the expense of submarines, some countries and corporations would probably continue to use "dated" technology.
Micromachines produce a wide range of naval toys, including submarines that can be used.
Scratch-building is another option, and the range of modern aircraft drop-tanks and other weaponry does provide some good basic building blocks from which to start.
There are a large range of SF space craft from a variety of manufacturers that can be utilised.