Although it may not be intuitive to folks generally sailing in a limited area but I'll make this suggestion again as I have when others asked similar questions.
There is nothing more comforting than calling for a tow and watch competing companies race to beat each other to the scene. You too can enjoy that by not signing a contract with any specific company but rather just add tow insurance to your existing liability policy and calling "any tow boat" on the radio if ever you need a tow.
Doing so costs only a few $$$ in my experience ( our underwriter charges us $8/year for the added endorsement) covers the entire cost of a tow and won't trigger premium increases just for that service. Additionally, the wait time will likely be less and for those of us who sail widely, the coverage is invaluable when only local tow operators exist.
Frankly, I don't know of any reason why anyone pays annual membership fees to a single service company.