You want a 40mhz set with a minimum of 4-channels, but preferably 6-channels or more, this will give you the flexibility to add more functions later on.
If you intend to expand your fleet a computer TX can offer some benefit, but not as much as some would have you believe.
Manufacturers I recommend are Futaba, Hitec and Multiplex. Be wary of unbranded 'cheap as chips' sets unless they have been thoroughly tested in the field. Submarines place extra special demands on reception quality so you want the best you can get.
Dual conversion receivers appear to be worth the extra beer vouchers.