Ho Outwest,
Iīm not flying the A139 but spent quite a time flying over water.
You shouldnīt worry to much about inadvertent inflation - just keep you fingers from the inflation-button....
But in case of an emergency, you might be so busy flying the bird, that you wonīt recognize, that you havenīt armed the floatation gear - until it is to late.
If you have a chance to do some simulator training the instructors will certainly be pleased to prove that to you....
Just stick to book - arm the flotation when passing the coastline.
Greetings Flying Bull