Well if it is caused by the rain I doubt it will be because of water in the tank - unless you have a serious problem water takes a lot of time to build up so a couple of days riding in the wet will unlikely cause that.
Damp can cause electrical issues but once it is dried out they should go away. Check the spark plug leads, plug caps and ignition cables are not damaged / cracked.
So it's likely to be a fault that, although it is worse in the wet, is a pemanent fault that is likely there but just may seem worse in the wet?
Is the mechanic a suzuki dealer? if so an ecu check could be done.
Like I say - trial and error I am afraid but if you check each component as you go along you will find the issue. Did you check the plugs - one of them could have a damaged electrode?
If you are convinced it is water related and have access to a hose pipe - let your bike stand ticking over then spray water over the plug leads and ignition components - if that does not cause the problem spray the ignition switch and all exposed electrical items on the bike i.e. switches etc. If the bike does not splutter etc, then it is not water related ;)