My hairline receded a bit - is it due to sebborrheic dermatitis? And any treatment to lower it? (Photos)

If the water temperature was hot enough to scald your skin or burn your skin then it may be an issue.  Otherwise it should not be an issue.

It is very difficult to correct a bad hair line after you had your surgery.  The only way may be to reshape your hairline LOWER than the existing hair line.

Cold temperature is not "dangerous" after a hair transplant surgery. Unless it is so cold that other parts of your body is impacted.  

It may take six to twelve months to appreciate the results of a hair transplant surgery.  If you are concerned it is best to contact your doctor and ask for the exact count of grafts you received. 

It is best to contact your doctor about PRE operative care and instructions since each doctor has a different regimen.It would be wise to have a clean hair/ clean scalp before surgery.

Fungal infection is not normal after a hair transplant surgery.  It would be in you interest to contact your doctor for an examination and receive proper treatment as soon as possible.