Usually it's part of the notice of race, but I think the Iowa crowd missed it.
Here's the one from the 16's:
Each Competitor must show proof of Liability Insurance with a minimum coverage in the amount of $100,000.00 (USD) with valid coverage for organized racing in the Northeastern United States. Competitorâ€™s proof of insurance and coverage must be received with the Official Entry Form. Competitors may purchase third party liability insurance onsite prior to competing in the 2006 Hobie 16 North American Championships. Anyone chartering a boat must purchase Borrowed Boat Liability Insurance at Registration.