Personally, id get myself an index marine junction box. (www.indexmarine.co.uk
You can then simply pass the cable into the box through a water tight cable gland, make the join neatly inside with connection blocks or crimps, pass the cable out the other side through another gland and off to the power source.
Screw the box to the transom and reaffix the lid and youll never have to worry about it again!
This appears to be how the professionals do the job 'properly' and it allows for modification at a later date as you can add more glands to the box and put other connections inside.
Also allows for peace of mind as you can take the lid off easily to check connections when doing maintenance - that way youll always know its fine rather than wondering if alls ok beneath that mound of sikaflex!!
Hope it helps