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I haven't done it and have no need to do it intentionally, but an opinion on a couple of your questions:
1. Endurance: Should be easy enough to figure if you want to test it by shutting one down at a comfortable altitude (overhead the airport?), establishing the speed you want for fish-spotting, and checking fuel flow. Alternatively, WAG it by figuring (conservatively) a 30-50% increase in fuel flow. I'm assuming you're normally throttled back pretty far while spotting?
2. Engine wear: Likely some additional from a cold start, but not much different from starting on the surface without preheat on colder days. I would let the restarted engine idle for at least five minutes with cowl flaps closed before s-l-o-w-l-y increasing power.
I don't know how much endurance you're ultimately looking for, but until a couple of years ago we had been flying 6+ hour marine survey missions with very comfortable reserves using full mains & aux tanks (100 knots GS on the survey tracks). Finally backed off to 4:30 max with full mains only for personnel comfort (pee breaks).
Joe
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