Okay, at this point, Magwitch is back in England. Pip and Herbert and Wemmick are hiding him. But they have a plan for helping him escape (because he will be executed if he is discovered back in England after having once been transported). So Pip is supposed to buy the boat as part of the plan.
The plan is that they will carry Magwitch away by boat. In the meantime, Herbert suggests, Pip should buy the boat and row around on the river a lot. The idea is that if people get used to seeing him rowing around all the time, they won't notice anything unusual when he actually takes Magwitch away.