when I make the drednought emergency land in the river I find it really hard to defend. there are too many boats and planes and I have no rockets left.

does anyone have any tips please?

what i should do is destroy the boats with your machineguns. is it really true that you dont have any rockets left? otherwise you could use these to shoot down fighters.
the second option is go fly in front of the drednought plane and use your flash blinder.

i hope you wil get trough this now

Since the roadblocks can be taken out with machine guns save one self guided missile to hit the fighter with the extra ammo tag after the checkpoint. Just shoot the missile and detonate it near the fighter and you'll get enough extra missiles. To kill the boats you'll need to shoot the missiles into the larger groups and also have Cowboy help you out machine gunning the stragglers. You have to work fast on this one. Good luck if you are still stuck on this.