Last year, most of my diving was fresh water, and in that water, I taught myself how to be a competent, qualified diver. All the fundamentals of weighting, trim, and buoyancy are universal (even if subject to slight differences in magnitude). Other aspects -- visibility, temperature,... drysuit management -- have been significantly more complicated in the freshwater sites.
This year, however, I've been spending significantly more time out in the Gulf on saltwater trips. The skills I've honed in fresh water have made the saltwater trips quite relaxing and enjoyable. The worst visibility we've had in the Gulf has been about what I'd consider a rather good day in many lakes. (So much so, in fact, that I was surprised when I was hanging out at the shop and heard the owner tell someone that the visibility was absolutely horrible. :biggrin
I've even had the honor of being complimented on my diving by our instructors (one who cave dives even said I had great form).
Still, while my freshwater skills apply just as much to saltwater diving, I have also noted interesting differences. In most fresh water, usually there's either a significant current (such as river diving) or basically no current. In some of the salt water areas we dive, on the other hand, subtle currents are often present. You don't really notice them unless you're paying attention, but they can complicate your dive, and when they're highly depth-variant they can really bring it sometimes. (A DM in saltwater needs to be more adept at reading the water.)
Another subtlety might be the problem of great visibility. When a diver can only see 20 feet or so, they're not likely to be far from their buddy, but when they can see 100 feet or more, they tend to stray. In lower vis, when they can't see their buddy, they know something's off, but in great vis, they may be too far from each other to be able to be of any help in an emergency, but since they can see each other so well, they never notice. (A DM in great vis has to be an external influence to keep the divers together, as the water doesn't do that by itself.)
And of course, there's the whole *boat* thing. The time I've spent out on dive boats has certainly helped me acclimate myself to the process of diving from real boats. Half of that is getting used to reading ladders and handling all your gear, but the other half for a DM is getting used to reading divers on a boat. It's just a whole different world when you compare boat diving (especially larger mass-diving boats) to shore diving -- everything becomes compressed on boats, as you're limited in space and things tend to all happen at once.
Anyway, for diving skills, I highly recommend quarries and lakes. They're great experience builders, and they'll certainly test *most* of your skills. A DM who has only been in quarries and lakes can easily have exceptional diving skills in form, navigation, and so on, but he will still be a new diver when he gets in the big tub. Acclimating to new diving conditions is like playing new musical instruments -- the first is hardest, and the more you add, the easier the next becomes. If you've been diving in a dozen quarries, lakes, and rivers, adding a new set of conditions that happens to have salt in the water shouldn't be *that* difficult (although I'm certainly not blockheaded enough to think it'd be a good idea for me to jump in off New Jersey or over at Monastery without learning the local knowledge).
(To quote NAUI's web page summary of DM requirements, "Dives shall be varied in environment, depth and activities." If you're only going to work as a DM in freshwater lakes and quarries, you can be perfectly adequate for that without having saltwater experience, but whatever additional experience you gain will only help make you a more valuable DM for those around you.)
