Somewhere in your boat policy is a clause describing the water you are covered on. It might be a distance offshore, a named body of water, a stretch of coast, or a combination. Outside it, you are not covered, and the clause does not announce itself.
How they are usually drawn
Common shapes are a stated distance from the coast, coastal and inland waters of a named state or states, or a region defined by latitude with seasonal conditions attached. Many Florida policies also carry a southern limit that excludes the Bahamas or the wider Caribbean unless it has been specifically added.
The seasonal element is the one that surprises people. A policy can cover a stretch of water for part of the year and exclude it during hurricane season, which is exactly the period an owner is most likely to be moving the boat somewhere unusual.
What to do about it
Read the clause before the trip rather than after. If a crossing or a longer run takes you outside the limits, most carriers will extend cover for the passage by endorsement, usually for a modest premium and usually with conditions about crew and season. That is an ordinary request and not an awkward one.
Two situations are worth flagging to us unprompted: buying a boat that was previously insured in another state, where the old limits may not describe how you will use it, and any plan to keep the boat somewhere other than its listed mooring. Both are cheap to fix in advance and expensive to discover in a claim.