What happened to compeyson after the fight in the water?

In chapter 54 Magwitch reveals Compeyson hiding on the boat and in the ensuing struggle both men fall overboard. Magwitch returns to the surface with injuries to the head and chest, and Compeyson drowns. Pip says-

'He [Magwitch] told me in a whisper they they had gone down, fiercely locked in each other's arms, and that there had been a struggle under water, and that he had disengaged himself, struck out, and swam away.'