Gee! you think maybe the location of the boat may play an important part in the cost of insurance? Or being in the south inside the Tropical Storm/Hurricane belt? As compared to way up north in - like R.I.
Anyway, as others have mentioned, the face value of the insurance and the liability coverage, etc., just like automobile insurance, can cause radically different prices for coverage.
If the boat is old and not of much value - AND - does NOT have a bank loan on it - then liability-only insurance can be a fraction of the cost. Liability only usually satisfies marinas
and boat yards for their insurance requirements.
It's all a matter of mostly "where" the boat will be used and kept. However, you should shop around. Try http://www.imiscorp.net/
, MD - they have a "Jackline" policy that is designed especially for couples and cruisers.