During the main tourist season (June - September) Bol is very well connected to the main land via ferry lines.
Most important one is Supetar (Brač) - Split (mainland). During the summer months it is a normal for a ferry to go up to 12 times a day (regular lines) and during Saturday, there could be few extra. First one from Split goes at 05:15 and last one at 23:59. Tipical ferry time is 50 min.
Besides Supetar-Split ferry line, Brač is also connected via Sumartin (Brač) - Makarska (mainland) ferry line. Ferry on this line goes 4 times a day in every direction.
If you are in Split and without a car, you can get to Bol via direct catamaran line that goes from Split every day at 16:30. It is a speed boat, passenger area is well air-conditioned and drive lasts for little over an hour.
More info on ferry boats can be found on Jadrolinija web.
Brač airport (BWK) is also a mean of transportation if you prefer airplane flight. Brač is in the main season connected to Zagreb, Oslo, Austria and other European cities so you can plan your trip from there.
More info on Brač airport can be found here.