I have both, and here is why.
progressive used to require you to pay the tow company out of pocket, then they reimburse you. up to a certain point. Now with sign & glide, they do pay the towing company directly, however its like AAA - you are not a priority for the towing company, where their members are.
boat US (and its rival SeaTow) are the tow companies, you wont see the bill, as they handle it internally. It may get sticky if you have to tow more than 25 miles as pricing escalates. And since they are towing other customers, you are usually no more than an hour for the boat to arrive.
I have been towed in 3 times, once was 23 miles. once was 1 mile, once was 8 miles If I did not have boat us insurance, that 23 mile tow would have been $1200 out of pocket at the time. because the price for towing is $300 an hour from the time they leave the dock to the time they get back.
for me, the boat US is a must if you boat in the ICW