Glad you found the leak. Yes, the 60-90-10 is a generic rotary shaft seal (aka lip seal), any brand is fine like SKF, Parker Hannefin, etc. and no, no special features are needed (ie., a stainless steel garter spring is not needed). You can get either a single lip or a double lip if that's all they've got (the 2nd lip is just a dirt barrier and can't hurt, but is not needed here).
This can most certainly be done in the water.
You may not have much success with the vacuum oil pump, other then emptying the reservoir, which you can do, perhaps more easily, by removing the reservoir's bottom hose from the C-drive's housing and draining the oil into a container. There will still be some dripping when you remove the old seal so just bunch some rags below it.
Hope I didn't put you off with talk of hand sledge and chisels-as-wedges - this is really a pretty easy job. You might want to see if it's in your DIY comfort zone by loosening the bolts on your Vetus coupling and seeing if the coupling is free to slide a fraction fore-and-aft on the C-drive shaft. That's half the battle. If it's free, then loosen the metal retaining ring that sits outside of the lip seal - it's got a small bolt clamping it to the shaft. If that wiggles free you're good to go. If those are frozen, maybe go ahead with your mechanic.
"Scooting" the engine back is just removing the four engine mounting bolts - no need to remove shift, throttle, fuel, water, or air connections - just four bolts. You'll want to use long extensions on your socket wrench and an "old man" extension pipe for the ratchet handle to give more leverage.
Then run a stout line between you primaries and through the engine's lifting ring, apply some tension to unweight the engine (don't pick it up - just unweight it), "scoot" it back and ... Bob's your Uncle.
The whole project should take 3 hours and 47 minutes. 😜
Good luck with it!
Craig SN68 Sangaris