Yes, the bridge is low enough at this point (the aforementioned damage puts this part closer to the water) for the fishmen to climb up easily and quickly. It is, however, only wide enough for single file. And the light you see is coming from far away (the island city), so without torches it wouldn't illuminate your view.
On the bridge:
f1 f2 f3 E P S f6 f7 f8
From the water, f4 and f5 are both holding weapons, spear an quarterstaff respectively. They are poised to attack from the water. The others are unarmed and attacking from the bridge.
Round 1:
Semabin (init 21) grabs his scimitar and attacks f6, slicing it for 4 HP damage.
f8 (init 18) can't reach anyone
Pendrake (init 16) is in the middle, Eanos and Semabin between him and the fishmen on either side. With a quarterstaff, f5 is below him. If Pendrake wants to make a melee attack into the water, that would require reaching over, thus risking falling in (DC 10 balance check to stay on the bridge, no taking 10).
Eanos (readied, new init 16) -- I'll adjudicate the rest of the round after getting Pendrake's action.
f3 16
f6 15
f2 12
f1 12
f4 11
f5 4
f7 4