Stuck in Route Kanal (by big underwater fan)

[Expat]

So I must be missing something obvious as no-one else seems stuck where I am, it's the section underground where you get the SMG and grenade and there's a room with a rotating underwater fan.
I can see a ladder out of the water but it's behind a gate and I can't work out what to do, I've tried blowing up the gate with a barrel & grenade but it has no effect, can anyone help please?

 

[antimatter]

Swim to the bottom of the fan, there is a small tunnel.

 

[Expat]

Thanks I'll try and find it, it's been driving me crazy.