There are so many issues around water. There is the need for clean water for creatures to drink and swim in for life. There is the need for enough water flow for crops and natural system cycles. There is the need for climate stability so that floods and huge rain storms do not kill too much.
I picked Paul Jenkin with the Surfrider Foundation because he is working to remove the Matilija Dam (he has been working on this for almost twenty years now! and he is almost done). By removing the dam he would restore the river inhabitants and habitats. It would restock the sands to the beach, it would mitigate a lot of the extra fertilizer agriculture is putting in the river. The silt that has built up around the dam can be used to fertilizer the area naturally and the steelhead trout will be able to go all the way upstream.
Paul's Facebook Page
I also pick Ben Pitterele with Santa Barbara Channel Keepers Stream Team. This teams works with local volunteers to monitor the Ventura river water quality once a month and send the data to the EPA. This teaches relevant science to any human who wishes to participate, lets us be well informed about our own back yards and sends up to date data points for GIS. I also really liked this because it focuses more on the animals and the ecosystem than how humans can benefit form it even though that is a side effect.
Ventura Stream Team
Ben's Facebook Page
I have worked with both of these guys and both of these groups. From coastal clean-up to monthly river walks to watershed habitat maintenance.