You will likely find that what you are doing will work best. You should get a little better rate with a company affiliated with your other insurances.
Just an FYI on your new engine warranty. Be sure you notice that the Mercury Gold plan warranty, normally sold in new boat deals, does not cover electrical components. The Platinum warranty DOES cover electrical components. Once you are on the backside of the new boat/engine warranty you may want to consider paying the cost to move to the Platinum coverage with Mercury. During the first year, on my 2017 Mercury 150, the alternator failed and had to be replaced. It was covered under the new purchase warranty but that occurance prompted me to go back and upgrade so that I could extend coverage on all components out to 5 years.