Caveat: this is coming from a guy who knows what he doesn't know and I don't know a lot about this topic, but I'll venture a guess and offer a possible direction for further inquiry on your part.
The survey is usually required so the underwriter can be sure the boat is worth what it's being insured for, and is in a condition that it's not going to sink once the lines are let go. Apart from the boat sinking at the dock and spilling fuel into the surrounding waters or the mast falling down and crushing someone walking down the dock, I'm curious as to why the condition of the vessel has much to do with liability for damage done in a marina. The reason the marina wants you insured may be that lawyers working for people who have sustained losses look for people they can blame/sue who have deep pockets. If you don't have insurance, the marina is next in line to be sued. They don't like that, and neither would their insurance company, which may be why there's a requirement that you have personal liability cover on the boat, i.e. the marina's insurer requires them to require you to have liability insurance so they're not the first in line when a law suit is filed.
Personal liability involved in taking people out in the boat is another matter -- the condition of the vessel has a direct impact on this. The marina isn't involved in this type of liability, but it may be that insurance companies can't separate the two types of liability -- i.e. that in the marina setting, and that once underway.
Talk to your home owner's / renter's insurance broker/company. You may find out that you're covered as far as personal liability is concerned up to the limits on your homeowner's / renter's insurance.
Others here, more knowledgeable than I, will certainly offer better insight and advice.