Having an ignition? problem

[tongy66086]

Hi all.....new member here and hoping one of the experts here can help with a problem I have.
I recently bought an 87 Moto 4 200. Had it out last weekend, and playing around in the fields. Ran it through a creek that went about half way up the tires....nothing very deep.
Rode it about another 10 or 15 minutes before it just died.
Towed it back to the garage, and checked the plug, and found no spark. Thought I had just splashed water up somewhere and it took a bit to soak in.
I figured I would let it sit a bit and the water would evaporate. It's been nearly a week now, and still no spark.
Could anyone give me some pointers at to what might be wrong and where to start looking?
I would have thought the ignition parts would have been sealed good enough to with stand what I had done.
Thanks so much for your help and suggestions.