Hello. Thanks for asking from HCM. I can understand your concern. According your history, you had injury over occipital region (posterior part of skull). The feeling of underwater sound could be due to some collection like blood and tissue fluid due to edema in scalp layers at the injury site. While you move, movement in collection leads to sound and sensation of underwater . For it, you can apply icepacks, avoid lying supine to prevent pressure on it, take anti-inflammatory pain killers like Diclofenac sodium after proper prescription to relieve pain and edema, keep your head at higher level than chest. If still no relief, it may be due to large amount of collection. You might need to consult doctor for evaluation. Now coming to your second complaint, it is not clear how diagnosis of cellulitis was made. But if it is present, you must consult your GP as soon as possible. The cellulitis treatment includes keep that area at higher level than chest, anti-inflammatory pain killers like Diclofenac sodium to relieve edema and pain, antibiotic sours like co-amoxyclav tab to treat infection. Cellulitis has a tendency to rapidly spread. Thats why it must be rapidly treated. Thanks. Hope it will help you. Take care.