Hard to accomplish with the wc3 engine. The best bet would be to give them a morph ability which makes them become another, amphibic, unit. This unit can then have better initial attackspeed / movement speed. Or just get a buff.
But checking periodically whether a unit is inside water or not would really tear down the performance of your map if you have plenty of those units to check.
Another way would of course be to place regions everywhere around water and check for entry/leave.
But that isnt all that nice either.
Set the units data so it can be targeted as decoration.
Copy the Spell and Buff for Endurance Aura. Change the names, icons, tooltips and visual effects of the copies whatever you like. Set the Aura Spell's targets allowed to friend, enemy and decoration.
Make a dummy unit with no unit model, no movement and no attacks enabled. Give this unit Locust and the New Aura.
Place neutral hostile copies of this unit in the river with enough frequency so the auras will all overlap and change aura size if needed. Anything in the river that counts as a decoration will get the speed buff.
While testing this I suggest you keep the Dummy visible to see where gaps in the auras are.
Strange to make him a decoration I know but this should eliminate any strain from triggers
You can do a every x.xx second, and check at the position of unit if the terrain is water, if it is, add Endurance aura (customize it as wanted, call it something like 'Water','This unit is on water, she's faster than usual'), else, remove the ability (Endurance aura)
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory
I will be AFK for a couple of days you guys hold down the fort while I am gone. I will be checking in on my phone but that will severly limit me. Be back saturday!
I just got to watch the video because even though I was there I was trying to work out tech problems with the chat since I was the social media guy so I was distracted but check it out guys the video is super fucking great!