Snce we have our home and auto policies with Allstate, I inquired about boat insurance for our up-coming purchase. You must stay within 100 miles of the USA (continental 48, I assume), but there was a rider to allow the whole of the Bahamas for a small additional fee.
Anyway, the estimated premium for agreed value coverage at $100K was less than $500 per year (that is discounted a little due to my Coast Guard Auxiliary membership and training resume). I believe it was for $1,000 deductible, but can''t recall.
Boat/US and Marine Underwrites both wanted well over $1,000 per year. I will investigate any differences further before making my decision, and so should you.