Chainmail Bikini's pdf book "Beyond Monks", written by our own James "Gargoyle" Garr, has rules for this!
The rule in this sourcebook is:
Hydroplaning: If your speed is 90ft or more,
you can attempt to move across water when
running by making a Balance check at DC 40. You
must have a head start on land or some other firm
surface of 20ft. or more. If this check is failed, you
immediately begin to sink and must begin
swimming to stay on the surface. If the check is
successful, you may move up to half your running
movement rate across the top of the water. Since
you are running, you must move in a straight line
and you lose your Dexterity bonus to AC. To
continue hydroplaning, make a Balance check at
DC 40 at the beginning of each of your turns.
Seems balanced to me.
If you want it shoot over to rpgnow, linked off of our main page. I really liked it, and for less than $10, I think it's a good buy.