The truth is, Israel has 5---as in five---attack submarines crudely hollowed out so that each houses 1---one---Harpoon cruise missile with a nuclear warhead. Additionally, they have a number of gravity nuclear bombs, also of antiquated provenance, designed to be dropped by F35I aircraft.
Let us analyse these threats separately. The five missiles have a range below 200 km; they cannot reach Iran from the Mediterranean, so that these non-stealth, diesel subs would have to trek around Africa (refueling where?) into the Gulf of Oman at least, in order to deliver their warheads to coastal Iran; Tehran is beyond range from any body of water*.
source:
https://www.nti.org/analysis/articles/israel-submarine-capabilities/
The obsolete gravity bombs will need to be flown to Iranian interior on aircraft, their range is negligible and they are not suitable for equipping with planing modules. This will require the US to explicitly handle the refueling and massive escort for these planes, which are no longer stealthy to Iranian AD. Also, their taking off from the Negev with the nukes will be observed by Chinese and Russian surveillance hours ahead of time, and all the AD will be mobilized; there will even be hours in which to partly evacuate Tehran, and of course to retaliate with all they've got, no holds barred---desalination plants, sacred cow Hassidic communities, Demona, all key infrastructure will be targeted.
The chance that one of these bombs will be successfully delivered is slim, but not completely negligible. Iran will be hurt, but not knocked out; Israel will become nonviable. That does not mean they will not do it; they are way past rational.