There are two major things one would considering insuring. First is the value of your boat, should it be damaged or lost. You make it seem like your investment in your boat is not worth insuring and that's your choice.
The second is the liability you would have, if you damage another boat or the marina itself. The biggest liability that concerns me is environmental:
fuel spills, chemicals, salvage/recovery. This is likely the one that the marina requires and regardless of the size of your boat, you should want as well.
There should be a local insurance agent that is experienced in these policies. You can get the cheapest, lowest cost policy available. However, I highly caution that you shop for value, not just the lowest price. Why have insurance that doesn't cover anything?
It should not be terribly expensive for just liability coverage, depending on your sailing resume. Hull coverage is another story and depends on age, condition, etc.