Is it correct to assume that you will arrive on the Plymouth and Brockton bus out of Boston on the commuter parking lot in Harwich? If so, your best bet is that you speak to someone at the B&B where you will stay to see if they can make arrangements to pick you up at the bus stop as it is not an in-town bus stop with much around it.
You may find having a car useful. If you decide renting one, Enterprise in nearby Orleans will pick you up where you are staying and will drop you off after you have returned the car, free of charge. You could ask them if they also meet a bus, however, waiting at the bus stop would not be my first choice, I'd rather go the B&B first and wait for the car rental people there.
Local transportation is provided by the Cape Cod Transit Authority www.ccrta.org. The Flex bus services Harwich and Harwichport, going to Provincetown and making stops in interesting and beautiful towns along the way.
Given that the bus from Logan ends in Hyannis, and then you'd have to use another one to get to Harwich, why not arrange for rental car in Hyannis?
While it is technically possible to get around w/o a car depending on where you are staying/going on Cape, a car is almost required to fully enjoy the Cape...I have only used the bus to get to/from Hyannis when I had car in repair shop in Hyannis and it was tolerable for those 2 trips but not something to enjoy.
Definitely get a car on the Cape. Our public transportation system is okay at best, but I would not recommend it for someone coming for a visit.
Although taking a bus might not be too palatable to some, it is the easiest way to/from Boston. Be aware, both Peter Pan/Bonanza and Plymouth & Brockton bus provide service to the Cape. (You will want to go to the Hyannis Transportation Center.) Plymouth & Brockton has, by far, the more extensive schedule.
Check car rental agencies at Hyannis Airport (airport code HYA). It is a small, regional airport just a few miles from the Hyannis bus station. Perhaps contacting the innkeeper at your B&B with your questions might be worthwhile.
If you are taking the bus to the Hyannis transportation center then Enterprise will pick you up there and bring you less than 1/2 mile to their office to get a car. They have two airport offices. One is considered off-airport and that one has lower rates and shorter hours.
Thank you for your help!
If you will be in Harwich, as opposed to Harwich Port, you will definitely need a car...not too much going on in Harwich center and you will need to go quite a distance to get to the water. Rent a car in Hyannis..you'll be much happier.