What makes marine aquarium water smell?

To begin with, the ocean it self has its signature smell consisting of sulphur, that is the smell of the ocean!

For our marine aquariums, ammonium-ammonia is always the strongest scent; nothing else will ever be as concentrated from your aquarium producing a scent as this action.
If something is dead, there is your smell.

The smell is from bacteria during the conversion of organic nitrogen from dead tissue to ammonium and onto ammonia, this is a fast action of ammonium to ammonia and is where the stink lies.

It ends up more so the toxic ammonia due to salt waters usual Ph and amonium isn’t  harmful.
If it was a sulphur smell of that degree, you just killed everything and performed a small miracle!
Both ruin PH,but it is always ammonia that you can smell.

Ammonia is a bi product of the initial nitrogen cycle bacteria and from that bacteria will come the oxidisers that take it to nitrate, its fool proof if the saltwater is made right.
So if anything like live rock is added, even live rock that’s been dead for some time left out of water or the needed flow, the nitrogen cycle will happen even if you do not add purchased bacteria.
The other way is to use fresh salt water from the ocean, this should be used with in 12 hours from collection; it has all that is needed to start the cycle.

The ammonia needs to spike and then be testing at zero before adding anything.