You actually don't find it in the great marsh. You have to clear the psyduck out of the way to get to celestic town once you are in celestic town you will deliver the Old Charm ,that Cynthia gives you when you cleared the psyduck, to Cynthia's Grandma which in return she gives you the HM surf. To figure out how to clear the psyduck I suggest you go here http://wiki.answers.com/Q/Where_is_the_5th_gym_leader_in_Pokemon_Diamond That also tells you where to find the 5th gym leader. I hope that really helped.
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You can find the HM surf in the Great Marsh in Pastoria City and then you can use it outside of battle once you beat the Pastoria Gym Leader.uhm, no. you have it wrong. you get defog in the great marsh safari, and can use defog after you beat the gym leader. is a town top of hearthome west up town up is it i am not saying the fird gym
If you are talking about Defog(HM05), then you will have to beat Crasher Wake first in order to use it outside of battle. It is somewhere in the Great Marsh. In Platinum, it's Fantina, and after you beat her, you can use Surf.
Surf is HM #3. :-)
you get it at the great marsh and talk to the green haired guy
you get from alder at twist mountain.