things i keep a spare of - remember they call boats a hole in the water you throw money into:-)
1) Extra Prop
2) Shaft seal packing (if you have this style of seal)
3) Extra Alternator
4) Extra Starter
Not worth losing a day of riding for the cost of these parts in my book - just call it cost of owning a habit. All of these things you can change yourself in less than the time it takes to drop off your boat at the dealer, if you have a few simple tools and a few mechanical skills. Plus you can find these parts for a lot less than your dealer will charge for them and sell them with the boat when you sell it - it adds value. Anyone who has had a boat and uses the heck out of it knows that these parts are the critical ones that usually go out.
I also suspect the prop myself. One thing you can check to see is if your prop can be spun by hand fairly easily while the boat is OFF, in neutral and on the trailer. If it doesn't it could be something in the alignment of the shaft to the tranny - either way like the guys say here - take the thing in pronto, the more you run a wobbly prop or shaft the quicker you start to spend more money on other parts - like shaft alignment, shaft seals, transmissions)