Is it really necessary to replace the stock exhaust valves on the LS3 with inconel ones?
Exhaust it is from Kodiak Marine.

Depends what you're doing with it. Keeping it stock, putting it in a small boat - probably not necessary. Adding a supercharger and pushing a turd at top speed for a few minutes at a time - probably is necessary.
Plenty of black merc motors didn't have inconel valves.

As was stated it all depends on what you’re doing with it. For a racing application you probably want to upgrade them. I swapped a Ls in my fishing boat. It’s going to be trolling under 2k rpm’s most of its life. I had the same concerns as you, a friend suggested I install a vacuum gauge so I can keep an eye on how hard the engine is working and with a chart you can see engine health really easily. I hooked mine up to the brake booster port and adapted it down to size I needed for tubing. Currently running the TBSS intake.

Typically the engine rev at around 4,000-4.500 rpm for 0,5 to 1,5
hour intervals and only occasionally briefly at 5,500-5.800 rpm.
I am especially afraid of possible water reverse or steam that could destroy classic valves. Resonable?

so if I read this right you are worried about reversion vs the valves operating under heavy load for extended time.
Reversion won’t just kill your valves. Whatever water doesn’t combust and turn into steam… if it’s really bad the leftover water doesn’t compress. In the compression stroke you have all sorts of carnage. Pistons, connecting rods, block, valves, pushrods, heads, etc.

the keys to avoiding reversion on LS swaps as I was told:
1. make sure you avoid certain auto cams. I’ll do my best to explain cams. There are others here that have forgotten more than I’ll ever know. But New hot cams today have big numbers caused by both valves being open at bottom dead center, and creating back pressure (vacuum) this vacuum will pull water in exhaust back into the engine.
there are Ls marine cams out there but make sure you have the right exhaust.
I spent hours in wait time and talked to several shops and got the same answers. Avoid cams with a lift duration around .224
Found one guy who’d done this before and recommended this setup. he told me don’t build a sports car, build a tow truck. That really helped me.
https://www.michiganmotorsports.com/...-2013-cam-kit/

I installed that kit in my LS headed for my boat. It’s been on run stand with the whole setup raw water, closed cooling, out the exhaust for a few hours to break it in and check function. So far no issues at all.

2. make sure you don’t need spacers on the risers. Same idea… you don’t want reversion. I had to custom mill some. I thought aluminum was such a weight saver but pcm or crusader would’ve been much easier. Of course there are very fancy setups for the unlimited budget.

I said the same things you are but my aviation engineer buddy and other builders assured me if I set it up like I did. I’d have more likely risk of burning my valves from load than reversion if I had good gaskets and tolerances accounted for. Only time will tell but I have several hours on it and so far so good. Gonna be putting it in as soon as weather improves enough.

dm me if you want to call lots of things I had to figure out on LS swapping.

Thanks for the great explanation.
I'm not afraid of reversing. On the Internet, I read opinions that the marine engine must have exhaust valves made of inconel due to a heavyer engine load, and examples were given, such as e.g. You go to a long fast drive and reach the coast at full throttle. Here you stop quickly and turn off the engine. Exhaust and engine heat pulls water vapor through the engine and hot exhaust valves can crack.
Theoretically, it can happen that when stopping, a drop of water from the stern wave splashes into the hot valves...

Is there anything to it?

So far, everything is fine, but if necessary, it is better to act now than to replace half the engine later.

Well, I can’t promise you it won’t happen. I stayed with the factory valves. I did clean them up and give them a valve grinding job. I ported my heads on top of the upgrade I listed above. In my opinion, you’d be better off to do a trunnion upgrade.
there are steam ports in the heads to avoid this issue from gm. I added a much better steam venting system to try to mitigate this. In addition I have a 6” spacer between my elbows and risers and flappers on the transom. I don’t intend to y pipe it but I’ve toyed with captains choice exhaust systems.

I don’t plan to run it red line. I will go a long ways from time to time tho. As it is I’ve run it quite a while on run stand and temps stay low. If you have the budget and want to go for it. But do you have too… I don’t think so. I tried to find the specs on Volvo pentas LS based engine and crusader 6.0 but haven’t located the valve specs. I suspect they are factory tho. But if you sleep better and can afford it sure…