Hi and thank you for your question.
As I understand you, your ski started and ran fine one day, then the next day it would not even turn over, much less start. If that is not correct, please let me know where I am wrong so I don't steer you in the wrong direction.
Now, assuming I have got that part correct, I would like some more details please.
First, were you riding in fresh or salt water the last day it was running?
Do you trailer your ski to and from the water or do you keep it in the water or on a davit?
Did you do any preventative maintenance or cleaning on the ski after you rode it for the day? If so, please tell me everything you did to the ski after riding it.
Have you tried removing the spark plugs to see if the engine will crank over without the spark plugs installed? IF you are going to try that now, make sure you ground the spark plug leads properly and do NOT have your face over the top of the engine in case it blows something out of the spark plug holes. You could even lay a heavy towel across the top of the cylinder head to help prevent spraying liquid around.
Are you checking the battery with a volt meter? IF so, how many volts does it have?
Next, if it has sufficient voltage with no load on it, have you tried a voltage drop test when you leave the volt meter connected to the battery and crank the engine and read what the voltage is while under a starting load, provided it is actually cranking, which I do not know at this point.
Is the meter lighting up when you power up the ski?
Let me know about those things and we can move forward with a diagnosis and repair from there.