Speaking of Progressive...I called them while looking for an ACV policy for an older car which has no "book" or "fiar market" value. Eventually the phone rep said their policies are written as "ACV or...whichever is less" and while they'd charge based on the agreed value, that wasn't necessarily what they would pay. Not impressive.
I'd call IMIS and ask them if you faxed up the BOATUS quote if they'd compare it line for line with you. They are usually very up front about differences in coverage and they know that sometimes they can't beat the competition. If there is something they can do to match the coverage, or other differences in the policy, they'll tell you.