My Marill won't forget HM surf on Pokemon Emerald ?
And yes I have taken it to the move deleter in Lilycove but the man says :
"Hm. It seems Beachball (Marril) doesn't want to forget Surf."-Or something like that.
Do any of your other Pokemon know Surf? If not, that's why. If Marill is the only one in your party who knows Surf it can't be forgotten. You can help Marill forget Surf by teaching another Pokemon Surf, and then go to the Move Deleter in Lilycove. This should work.
Source: http://www.gamefaqs.com/boards/921905-pokemon-emerald-version/50021203 and http://www.gamefaqs.com/gba/921905-pokemon-emerald-version/answers?qid=51374
Hope I Helped!
surf is a HM and the only way to forget it is through a move delete. (I have not played emerald)