mercruiser 6.2

[phantompete]

Hi all,
I'm hoping someone might be able to give good advice about
this problem I'm having with my boat.
It's a 2004 Hydrolift S24 with merc 6.2.
Over the summer it would occasionally be difficult to start,always
did though and once running was fine.
Now however it's got quite bad,it took me half an hour to get it
to go the other day,cranks fine and tries to start just as I release the
key.
I am an experienced car mechanic and I am wondering if it uses a ballast
type ignition system where the coil gets 12v when cranking but actually runs
on 9v?
The boat is very low hours and have had a bit of trouble with various switches failing due to lack of use,could it actually be the ignition switch?
I have already fitted a new genuine distributor cap and rotor arm but no difference.