Yes.Do you have and use piston 1sts?
Yes. A Mk10 has - quite seriously - 5 "significant" parts (if you count the swivel/turret as one) - the body, turret/swivel, HP seat cap, piston and spring. It also has about a half-dozen O-rings in it. all of which are replaceable with generic EPR 70 duros. Not including the cleaning (ultrasonic tank) and drying time I can strip and rebuild one in about 10 minutes.Are they that much easier than diaphragm 1sts to service yourself?
Typically, yes.Are there less parts in the 1st stage?
Not where it matters. Yes, there are some minor rust spots on the spring when you tear them down. And if you really abuse one, the body can get pitted, but that's true of any first.Doesn't salt get into the 1st stage and cause corrosion?
If you get saltwater INSIDE, it will get trashed, but that's true of ANY reg. So don't do that ALL regs have to be soaked in fresh water after use if you want things to work well over time. All.
I've got a bunch of Mk10s, all of which are many years old, none of which I am the original owner of, and while there is some superficial external corrosion and even a couple of places externally where the plating is damaged on a couple of them, there are places where there's some minor corrosion that has happened on the threads between body parts (not "internal" - would have happened with ANY reg) and some have come to me looking like they were buried in a sand dune somewhere, I've yet to have one fail to work properly, leak, be impossible to rebuild and restore to proper operation or otherwise give me trouble operationally as a consequence of it.
I've also yet to have one have a catastrophic seat failure. Generally as the seats age they will suffer quite a bit of creep but even with a BADLY trashed seat they'll seal - it'll just happen at 10 or 20 psi more pressure than normal.