Go outside, activate the bridge, and jump in the water. It might be a good idea to go fishing first. There is a door under the bridge. Inside, on the right, is a big pipe. Jump over, a secret is hidden behind it.

Get back in the water. There is a red 'conductor' next to a propeller. Shoot it, the propeller stops. Then jump in the pipe, swim to the end, and you have #2.

Safely arrived at secret #2, go in here. On the rear side of the pipe is a enterance to a secret level: 'Comm Satellite'.

This doesen't count as a secret, but it's really handy...
On your return from 'Comm Satellite', don't jump in the water to get out again. Climb up the ladder to the small cave. Shoot the back wall next to the corpse and enter the corridor - this is the opposite end of the bridge. Now all the baddies stand with their back to you... ;)