Between BW2 and BW3 Marlon had a run in with an angry group of Sharpedo which is the result of his scars. His mother Oceana traveled back from Kanto where she was teaching swimming lessons at Cerulean City to help take over the gym while Marlon got better. When he was fit to battle again Oceana decided to stay and battle with him to help half the work load and because she had come to enjoy it. The Pokemon Marlon used in BW2 actually belong to Oceana so now he is starting over with a younger party.
Humilau City was hit by a Tsunami just before BW3 and they have moved everything over to Nimbasa City where they are planning to open a swimming pool. The pool will act as an unofficial gym that you can partake in but there will be no badge prize.
>>16321341He must've been trained by Shadman.