If I spot the aircraft on time, I crash dive. If they surprise me and I dont have time to dive, I try to shoot him, and as soon as he's above me, dive before he turns back.
With the stock SH3, I manage to do
this 
. Once.
@Paul Riley
Regarding the use of the radar, I like to pretend that it's been thrown overboard, because turning it on will attract all RAF and Royal Navy directly at you.