I've found a boat I'm interested in, and it's a 1979, so finding insurance has been a bit tricky. I was eventually able to convince BoatUS to cover me
Any thoughts on why the premiums are so drastically different?
This reminds me of the Progressive commercials on TV where they claim you can choose the "policy (read - premium amount) that fits your budget. Never, repeat - never choose an insurance policy based on price.
Some of the other insurance threads here often compare premium amounts with no substantive discussion of what is included other than the typical agreed-value vs. ACV discussion which misses virtually everything one might reasonably expect to happen.
It should be obvious that the reason premium amounts differ is because they insure different risks, have different limits, different exclusions and different compensation policies.
About the best thing you can do is ask a number of underwriters for a copy of their policy. Read them. Take notes. Compare exclusions. Only as a last action should anyone compare prices as only after thoroughly reviewing what they are selling can you make a reasonable judgment of value (not price).