C-3PO
There's a very good description of the difference between the two types of servo here
http://www.rchelicopterfun.com/rc-servos.html You don't need to change either the Rx or Tx to use a digital servo but the caveat about having a larger-capacity battery pack or high-current BEC should be well heeded.
Andy
You may have failed to notice that I said 99% of model boat applications! After all, this is Modelboat Mayhem and there's always some contrary b*gger who wants to take issue - hence my bets are usually hedged! High-stress applications such as multi-boat and big offshore racers are clearly an exception as regards torque, but I still doubt that the difference between the speed of the two types could mean the difference between winning and losing a race. We're talking here about less than a tenth of a second end-to-end difference in transit time, and I'm not convinced that anyone could react quickly enough to take advantage of that tiny fraction of a second - except for maybe a Formula 1 racing car driver or a fast-jet pilot.
That said, these boats aren't festooned with a bank of digital servos like a model aircraft is, and they certainly don't rely on the BEC output from an electronic speed controller. It's the high current used which seems to cause the most problems, along with maybe poor battery choice and maintenance. Cater for that properly and you should have none.
Ballastanksian
A decent high-torque non-digital servo should be suitable for valves etc; they don't need the speed of a digital servo so why pay for it? As for the availability of alternatives, who would have thought even five years ago that we would all effectively be shunted over to buying 2G4 kit, even if we didn't want it? I sympathise with you submarine blokes, but you represent a minuscule fraction of a percent of all RC modellers - certainly compared to the hoards of flyers and car racers who think 2G4 is the dog's best bits and that digital servos go along with it (for some obscure reason). If the manufacturers decide to "rationalise" i.e. move over entirely to digital servos, then the days of 1A BECs and 700maH Rx packs will be numbered. Let's hope they don't.
DM