I don't see the exclusions likely impacting the average traveling diver that much.
What kind of costs would a foreign diver visiting California be faced with forfeiting
I'm not trying to be nasty, but my assessment is that you're thoroughly clueless and out of touch with reality.
We could argue about the meaning of "average", and it's possible that the 120' (maybe it's supposed to be 130', which fits the standard recreational limit and conforms closely to the 40m the rest of the world uses) and DM exclusions wouldn't affect 70 or 80% of divers, but they're definitely going to affect a substantial percentage of serious divers. If I go to Grand Cayman most ops insist that wall dives be done with a group led by a DM, but the DM is usually happy to turn you loose when you're back up to about 60' or thereabouts. The 2nd dive is a shallow one, and I've never encountered an op that won't let you dive on your own (with a buddy). Shore diving on GC is quite common. At CocoView, in Roatan, there are two boat trips per day. Each starts with a dive lead by a DM, and then they'll drop you on the wall near the resort for a 2nd dive on the way back. The average diver at CCV will be doing half of their boat dives without a DM. Then there's the very easy and unlimited shore diving; some guests don't even bother with the boat dives. My personal record is 7 dives in one day, 5 of which were without a DM. Some liveaboards will run group dives, and on others you can go on your own whenever you feel like it. There's a big world out there that's apparently beyond your experience where there's a lot of diving being done without a DM.
As for costs, if you miss a week of pre-paid diving and can't use a week of pre-paid lodging you're losing something you bought, but there's not really a financial loss. The problem is that you can incur a bunch of additional costs as a result of an accident. If travel dates change you'll at least be paying an airline's fee to change your flights, and you'll usually be paying a higher fare for the rescheduled flight, typically for you and at least one traveling companion. You may end up in a different location where you need new accommodations. Then there's the possibility that you need medical treatment that's not available where you are. Maybe the nearest recompression chamber is on a different island that's hundreds of miles away. That means flying there at a cabin altitude that's as close as possible to sea level, and that means a charter flight. That charter flight might be a real air ambulance, and could cost many thousands of dollars.
And I have NO coverage if I am driving a 4wheel drive
If it just says no coverage when driving a 4WD that illustrates one of the problems with insurance companies. So do several of the other things posted above. I've got no quarrel with an exclusion if you're actually out 4 wheeling some place, but most driving with 4WD vehicles is done on dry, paved roads. The vast majority of people who go to St John (USVI) rent 4WD, and if they go off-road it's only because some villas are on unpaved, and somewhat rutted roads. That involves some minor risk you wouldn't have elsewhere, but it also reduces your risk of a 55 mph head on collision to roughly zero.
They're total BS that should be tossed out in a lawsuit. How is "a high level of stress or risk" determined, and by whom? If it's up to the insurance company it's a get out of jail free card so they can deny coverage for a host of things that really don't entail significant risk.