I second doing some research on which company has a base closest to your sailing area and going with them. Look at the plans and buy the coverage that makes sense to you. BoatUS has different coverage including one level that will provide a two out of your home slip if needed. Usually the policy will only cover towing from a dock if you are a transient there. If you think you could get your boat from the slip to the maintenance dock by other means, it might not matter, but if you find you can't get the boat to start and need it moved some distance, it might be good to have.BoatUS is an excellent value. Their unlimited package is not that expensive compared with the cost if you need help. I have not checked on the prices for SeaTow.
In the Chesapeake, both BoatUS and SeaTow will probably do the job for you. If you intend to make trips outside the local area, you may want to check on their coverage as it varies.
I have coverage with BoatUS and the one time I needed them, their response was outstanding. The answered my 2nd hail, said they be their in 30 minutes and exactly 30 minutes later I saw the Red Tow boat come around the point. The operator quickly and efficiently took is in tow, then when we got into protected water, took us "on the hip" and had me steer us in to the maintenance dock. The tow was completely covered under my unlimited policy. The total bill was several hundred dollars which was eye opening because I had sailed myself to the just outside the creek entrance so the tow was short and didn't involve even a soft ungrounding.
We now have a Sea Tow franchise based closer to my marina and I may switch but it won't be because I'm not happy with Towboat US.