The problem is not the cost of an insurance policy, it's the requirement to get a "survey". IMO, this "survey" procedure ought to be reserved for people who want to buy a boat and find out if there's anything wrong with it, not as a requirement to get insurance. It is just too costly to pay someone $700 to come and look over an old boat to confirm that, "yep, it's a boat." Companies do not seem to be able to use photos or send people of their own to look at a boat like they do with houses or cars. Instead, they expect you to pay this outrageous cost so you can then be qualified to pay them some exorbitant amount. This antiquated procedure is a circle jerk.
Most insurance companies will want a survey/estimate on classic cars as well, it's not just boats. But yes you are right, they don't require anything but pictures for a house or a new car...it is weird isn't it.