As you probably already know steel isnt your friend direct to the sea.
Timber is much more resilient in that environment.
To meet Australian standards within 1km of rolling ocean your standards in steel will be very demanding on the Wallet.
At a guess I would say 300mm high x 10mm high UB would be needed to span 10 metres fully double dipped galv.
Being I-beam may reduce the need for cleats welded on the back to take the purloins and other beams placed.
If you Use C-channel then you will have to weld cleats on the back.