The company that underwrites our larger boat is Zurich Insurance. They added my Triumph 150CC right to my yacht policy (their name for it, not mine) as a tender to the larger boat at no additional cost. Their policies cover tenders 15 feet or less with a 50hp engine or less. The 150CC w/a 50hp Yamaha just fit the bill.
They are resonable, quick and have great customer service, should you need it. My broker is the Pier Group out of Trumbull, Connecticut, but there are a lot of other brokers that sell Zurich.