Hi and thank you for your question.
First, Yamaha didn't start making water vehicles until 1989. The first year of the Wave Blaster was 1993. If we need to get too specific, I will need to know what you really have as far as Make, Model and Year.
However, there are some generics we can check in the mean time.
First, what do the spark plugs look like when you take them out? Does the tip have heavy carbon deposits? Is it black and oily? Does it seem to be damp with gas? What do you see on the spark plugs after trying to start it?
Next, have you done a compression test? If not, you need to do that now. To do a compression test, only pull one spark plug at a time, and make sure you ground the spark plug leads appropriately. Usually the best thing to do is use the one spark plug you removed and put it back in the plug cap and lay it on the head, then use an extra spark plug for the other spark plug lead and ground it to the head.
Let me know what you see on the spark plugs and what you get for a compression test.