With daysailing and coastal cruising within the U.S. to be your initial use of a boat, I would suggest that you first contact your home insurance company for a quote. Be sure to read all the conditions. For example, what are hurricane (or "named storm") rules and restrictions? They might be comparable to Florida's restrictions, so it's a good idea to find that out first.
Also, ask some of the other boatowners in the marina you would be keeping the boat in for who they are buying insurance from. Sometimes the local agent makes all the difference in the success or failure of your insurance experience.
You can also seek out insurance brokers and agents who provide boat insurance, such as BoatUS, which usually has quite competitive prices.
You will not know how much your insurance premium will be until you know the make, size, and age of the boat you will acquire. Generally, but not necessarily, insurance companies require a survey for any used boat, and particularly for any boat more than 10 years old. Of course, you should have a survey done when buying a used boat anyway, so it would not be an extra expense.
As a rough guess, boat insurance will cost approximately 1.5% of the insured value. That is a very rough estimate; my experience with insurance prices is limited to our two boats, and those boats are not comparable to what you are looking for.
Good luck in your search!